Young Merlin, sent by his mother from their village to start a better life, arrives in Camelot where he is to be apprenticed to Gaius, physician to the repressive King Uther. Uther, believing all magic evil, has made it punishable by death, so when Gaius finds that Merlin has magical, telekinetic powers, he agrees to keep the boy's secret. The mother of a sorcerer Uther recently executed comes to Camelot for revenge in the guise of a pretty girl, charming the court to sleep with her singing. Only Merlin sees what is happening and, staying awake, saves the situation without exposing himself as a warlock. As a reward, he is promoted to valet of Uther's arrogant son, Arthur.

King Uther Pendragon holds an annual tournament where Prince Arthur Pendragon is to defend his title against other knights of the realm. Sir Valiant comes with a magic secret weapon - a shield from which the painted poisonous serpents can come to life and attack (which Sir Valiant also used to murder its creator). By nursing snake-poisoned knight Ewan, Merlin finds out how this vile knight has been defeating adversaries as easily as Arthur. He informs the prince, but a servant's word is worthless against a knight's, and by the time the prince challenges Valiant, the only 'worthy' witness has died (via more serpentine magic). Arthur, furious to be humiliated and seeming scared, fires Merlin. Receiving enigmatic encouragement from the Great Dragon, Merlin decides against giving up, resorting to magic from the book Gaius earlier gave him.

The dark sorceress Nimueh casts a spell on a dragon egg she throws into the castle's water. The next day a plague causes people to die, their skin turning white with dark blue veins. When King Uther orders Prince Arthur to search all Camelot for the sorcerer he believes to have caused the epidemic, only a magic trick saves Merlin from having his book found. When Gwen's father exhibits symptoms, Merlin waves Gaius's warning by curing him. Arthur reports the recovery, and Uther orders her execution at the stake. Merlin tells Gwen's caring mistress, Morgana, only Arthur can help him fight the monster (an afanc), which they search for in the dragon's cave.

In poisoning a cup, Nimueh's latest scheme jeopardizes a peace accord between two kingdoms, turns father against son, lays a foe low, and sends Arthur on a perilous quest alone.

Merlin would have been killed by a giant griffin (half eagle, half lion) if not for valiant Lancelot, who bravely attacked it and got them safely away. Learning Lancelot's life dream is to become a knight, grateful Merlin brings him to court, but when told only noblemen may apply, magically forges proof of nobility. Lancelot passes hazing and close combat tests against Prince Arthur but is found out as commoner and jailed. When the griffin attacks Camelot - even inside the royal castle - the knights' arms prove unable to penetrate it . Arthur frees Lancelot, who in return (with Merlin's magical help) triumphs where all knighthood fails miserably, but will Uther finally hear Arthur's plea to allow the best sword to serve in chivalry despite existing social restrictions?

Receiving tainted lilies collected by unsuspecting Gwen, royal ward Morgana falls ill with an unidentified brain disease Gaius can't cure. Mysterious healer Edwin's "remedy to cure all ills" seemingly works (by actually removing the cursed black beetle he hid in the lilies). As Edwin maneuvers grateful King Uther to instate him as new court physician, Gaius gets court archivist Geoffrey to allow him illegal access to the sealed records of Uther's purge of magic, revealing Edwin's parents as executed black sorcerers. After consulting the dragon (who says he must choose whether or not Arthur and Merlin's destined time has come), Gaius, taking a threat by Edwin to expose Merlin's magic, tells Merlin - the "son he never had" - that the time has come to step down. Edwin, now free to use another beetle, condemns Uther to an agonizing death like his parents but takes one monumental risk - a duel of magic with 'mere novice' Merlin.

When Arthur becomes besotted with a mysterious girl, he stands to lose more than just his heart.

When a druid boy's father is caught by the king's guards, the boy sends out a telepathic call for help, to which Merlin responds and offers sanctuary; but, under penalty of death to anyone harboring the boy, will Merlin be willing to put himself and his friends in mortal danger? And when Uther's dungeon dragon hints of dark prophecies linked to the boy, will Merlin then turn a blind eye?

A black knight crashes Arthur's coming-of-age ceremony, throwing down the gauntlet to any knight who will fight him, killing all who do, one day at a time. Gaius discovers he is Uther's dead brother-in-law, now a wraith summoned from the grave by the witch Nimueh, seeking revenge for the death of his sister who died giving birth to Arthur. His intent: to eventually fight and kill Uther. Merlin learns that only a special sword burnished with a dragon's fiery breath can kill the knight. To that end, he asks Gwen for the best sword her father ever forged (the sword later known as Excalibur) and asks the imprisoned dragon to burnish it. The Dragon, however, tells Merlin that, once created, only Arthur should wield it, as evil will come of it otherwise. Merlin gives his word. Unfortunately, Uther has Arthur drugged and locked in his room, preparing to face the wraith himself. With Excalibur in his hand, victory might come but would leave Uther in control of a powerful magical weapon.

Hearing that his village is being threatened by a warlord and his gang, Merlin travels home with Gwen, Morgana and Arthur, the latter doing his best to drill the peaceful villagers into a fighting force. Merlin's friend William is dismissive of Arthur's efforts, feeling they constitute egoism, though he ultimately saves the young prince's life. Nonetheless the villagers are inexperienced in martial ways and Merlin must use magic and conjure up a wind-storm to help see off the bandits.

While hunting, Arthur and Merlin see a unicorn. Despite Merlin's protests, Arthur kills the beautiful creature to bring the crass Uther its horn as an ornament. The foolish father and son ignore Gaius's warning that it is bad luck to kill a unicorn, but soon the crops wither and the well water turns to sand. The mystic Anhora, guardian of the unicorns, arrives in Camelot to tell Arthur that his stupidity has cursed the land and he alone can lift that curse. This involves his learning much-needed humility before riding to the Labyrinth of Gedref, where he will be tested. Merlin follows him but, by doing so, places both boys in danger. Unusually, it is Arthur who saves the day with a demonstration of selflessness, ultimately restoring the welfare of Camelot and the unicorn itself.

Gwen's blacksmith father Tom is approached by Tauren, who possesses the alchemist's stone and wants Tom's help in turning base metal into gold - gold he intends using to overthrow the king. The palace guards raid the smithy; Tauren escapes but Tom is caught, sentenced to death for assisting sorcery and (unsuccessfully helped to escape by Morgana ) killed. Appalled by Uther's callousness, she tells Tauren she will help him in his plot, unaware that Merlin is listening. On the pretext of visiting the grave of her father, for whose death she blames Uther, she rides out with the king to lead him into Tauren's trap.

The Questing Beast is on the loose near Camelot, and Arthur, ignoring a warning from Morgana who has dreamed that it will kill him, is bitten by the creature before Merlin slays it. Arthur sinks into a coma and the Great Dragon tells Merlin he can only save him by traveling to the Isle of the Blessed. On arrival there Merlin meets the witch Nimueh, who gives him the antidote, water from the Cup of Life, but warns him that, to save Arthur's life, another must be sacrificed. Arthur drinks the water and recovers but Merlin discovers the witch was not lying - his mother is consequently endangered. This will lead to the greatest ever test of both his emotions and his magic powers as he returns for a last show-down with Nimueh.

A treasure chamber is unearthed beneath Camelot, the centre-piece being a glowing blue stone in the sarcophagus lid of Cornelius Sigan. Decades earlier, Sigan was executed for sorcery and died vowing revenge. Greedy Cedric, hearing only of the treasure, comes to Camelot. Learning that Arthur has the one key to the vault, he worms his way into the dim young prince's confidence and steals it, looting the chamber and prying away the stone (which is actually Sigan's wicked soul). Consequently possessed by Sigan's spirit, Cedric takes on his magic powers and animates the castle gargoyles, which terrorize the city. In return for a promise to one day set him free, Merlin acquires extra powers from the Great Dragon and does battle with Cedric, saving Camelot once again. Given what has happened, Uther is more convinced than before that magic is only evil.

Realising that his jousting opponents always let him win because he is the royal prince, Arthur pretends to leave court and enters the lists in disguise to win on merit. He stays with Guinevere, who tells him some home truths about his arrogance, and they kiss. Moorish assassin Myror, paid by neighbouring king Odin (whose son challenged a reluctant Arthur to a duel and lost his life) arrives at Camelot to kill the young prince. He enters the joust with a lance hiding a dagger on its tip, but Merlin uses magic to defeat and kill him. Arthur has learned some humility by his experience but tells Guinevere that Uther would oppose any romance between them.

Morgana realizes she has magic powers when she sets her room on fire from her bed. Against the advice of Gaius and the dragon, Merlin sends her to seek advice from the Druids. Uther, believing she has been abducted, orders the deaths of all suspected sorcerers and their sympathizers unless she is returned, so Merlin goes after her. Locating her in the Druid village where chieftain Aglain tells her the truth (giving her confidence that magic can be used for good), Merlin must get her back to Camelot before Arthur and a vengeful search party catches up with them and kills all.

Gwen is mistaken for Morgana and kidnapped. Against Uther's wishes, Arthur heads out to fight for her freedom, with Merlin by his side to help him.

Part One: An ugly old troll drinks a potion transforming herself into the glamorous Lady Catrina, who arrives at Camelot with her servant Jonas. Gaius, who has met the real Catrina, and Merlin, who witnesses her true identity, are unable to convince bone-headed Uther of Catrina's real nature, especially when she bewitches him by giving him a magic pendant. Merlin attempts magic to expose her but she is too strong and it is too late to stop her marrying the king and becoming the queen of Camelot.

Merlin spies on the troll and steals the potion she uses to resemble Catrina, replacing it with a fake one made by Gaius. Consequently she turns back into a troll, in view of everybody, but Uther, still bewitched by her, fails to see this. Merlin learns that the only way to break the spell on him is to make him weep real tears. Thus Gaius prepares another potion, which Arthur drinks, and makes him appear to be dead. This does the trick and the king finally comes to his senses. Arthur kills the troll and Jonas and thanks Merlin for his efforts.

In his determination to rule magic out of Camelot forever, Uther hires the services of Aredian, a terrifying witch hunter who is most feared due to the fact that he stops at nothing in the quest to uncover all and every kind of sorcery. Even though no one is under direct suspicion, several people in Camelot are afraid that the witch hunter could see straight through them: Morgana is worried that Uther might have her executed and Gaius is concerned for the fate of Merlin and his own, since caring for the young warlock is considered crime and treason. Will all three be able to escape a dreadful fate?

When a mysterious warrior challenges Arthur to a duel, no one could possibly predict the devastating chain of events it sets in motion - least of all the young prince himself. Arthur and Merlin soon find themselves on a strange quest at the behest of the beautiful Morgause. Through her, Arthur discovers a dark secret which threatens to bring about the downfall of the kingdom.

Merlin saves a Druid girl from a bounty hunter's cage, but there is more to her than meets the eye.

Rulers from the Five Kingdoms converge on Camelot for a peace conference, including King Olaf and his spoilt, over-protected daughter Vivian. King Alined is also present, along with his jester, the wizard Trickler. He is a disruptive force who wants to cause discord between Olaf and Uther to gain ultimate power for himself. To this end he gets Trickler to bewitch Vivian and Arthur so that they fall in love and incur the annoyance of Olaf, who will challenge Arthur to a duel. This time it is down to Guinevere, rather than Merlin, to break the spell.

Mordred brings a warlock, Alvarr, to see Morgana, and they persuade her to steal a crystal with magical properties from the vaults of Camelot. Alvarr tells her it can be used to help people like themselves with special powers, though he really intends to destroy Camelot with it. Merlin and Arthur pursue him, returning him and the crystal to court. Mordred escapes, and Morgana (having disowned Uther), springs Alvarr from prison by drugging the guards. Merlin, having looked into the crystal, is alarmed by a vision of chaos, but Gaius assures him that it is far into the future.

Using an incantation, Morgause revives the deadly Knights of Medhir and, whilst Merlin and Arthur are the sole survivors of the party sent to beat the knights, Morgause uses Morgana to bewitch Camelot so that everybody except Morgana herself falls asleep, rendering the court defenseless when the knights approach, intent on killing the king. Merlin learns from the Great Dragon that the only way to break the spell is to kill Morgana, so he uses her as his bargaining power to force Morgause to call off the attack. As a consequence he must honour his pledge to free the dragon.

Morgana has gone with Morgause, and the dragon, freed by Merlin, repeatedly attacks Camelot, causing huge fire damage. Only a dragon-lord is capable of containing the beast, men whom Uther persecuted years earlier, from which only one still survives (Balinor), who lives like a hermit in a cave. Gaius, who helped him escape Camelot, tells Merlin this is his father, and when Merlin and Arthur trace him, Merlin reveals himself to Balinor (though they keep it from Arthur). Balinor agrees to come to Camelot to assist, but there is further drama before Merlin proves himself to be his father's son and defeats the dragon.

A year after being whisked away from Camelot, Morgana is found, disheveled and bruised in the forest and claiming abduction by bandits. Superficially contrite and charming, she remains in league with Morgause, for whom she collects tears shed by Uther. These Morguase uses to curse him through a doll fashioned from mandrake root, which Morgana places under his bed. As it slowly starts to drive the king mad with hallucinates of ghosts, Morgause - after taking Merlin prisoner - induces rival king Cenred to invade the weakened kingdom, for which Merlin summons the Great Dragon for aid.

The Great Dragon returns Merlin to Camelot where, convincing Gaius of Morgana's treachery, they break the curse on Uther by burning the mandrake doll. Nonetheless, they feel he would not believe them if they exposed Morgana. Whilst Cenred's men lay siege to the castle, Morgana uses a rowan wood magic staff to create an army of sword-wielding skeletons to fight the knights inside Camelot. Merlin destroys the staff, the skeletons disintegrate and, as the tide turns in favour of Camelot, Cenred calls off his siege. Morgana, however, takes the credit for destroying the spell and is acclaimed by her foolish guardian.

Merlin finds a secret chamber and accidentally releases a Goblin, which wreaks havoc.

A heroic stranger causes trouble for Merlin after risking his life to save Arthur.

Fleeing enemies, Merlin and Arthur take refuge in The Valley of the Fallen Kings. There, the sorcerer Taliesin cures the wounded Arthur and shows Merlin the Crystal Cave, which provides a disturbing vision of the future in which Morgana stabs a sleeping Uther. Come her birthday, Morgana, given a dagger by Arthur, steals into the king's room to kill, secretly followed by Merlin, who uses magic to thwart her. She is temporarily put into a coma, during which time Uther confides in Gaius that she is his natural daughter - a fact of which she is also aware. Gaius believes that Arthur should be told as now Morgana is a legitimate heir to throne and he stands in her way.

Arthur is pressured into an arranged marriage with a visiting princess, but there's more to her than mere gracelessness and a taste for frogs.

Gwen faces an impossible dilemma when she is forced to choose between saving the life of her estranged brother (Elyan) or Arthur.

Arthur must journey alone to seek out an ancient sorcerer, the Fisher King, and retrieve a golden trident to prove his right to the throne. Morgana gives him an amulet, the Eye of the Phoenix, which will weaken and kill him if worn for too long. Discovering this, Merlin sets out after him, collecting Gwaine, and they save the debilitated prince from two attacking wyverns, discarding the amulet. Arthur believes the quest is his but Merlin, separated from the others, meets the Fisher King, who tells him he is the chosen one. He gives him a bottle of water from the lake of Avalon to use if Camelot is in peril. The quest is accomplished but Guinevere has also discovered that Morgana is a baleful sorceress and confides in Gaius.

A healer is in Camelot using magic to heal the sick, and Gaius quickly realizes it is Alice, a woman whom he was about to marry many years earlier until she had to flee from Uther's persecution of sorcerers. Now Gaius is happy to work with Alice again in preparing potions, but he is unaware that Alice is being controlled by a mythical monster, a manticore, which urges her to poison the cold-hearted king. To break the manticore's hold over Alice and ultimately save her from execution, Gaius has to perform the magic that he possessed but had forsaken long ago.

Prophetic dreams of Guinevere becoming Camelot's Queen torment Morgana. Morgause encourages her to reveal Arthur and Gwen's courtship to Uther, knowing the king will do whatever it takes to stop his son from marrying a serving girl. As a result, Uther banishes Gwen from the kingdom, forcing Arthur to make the ultimate sacrifice of leaving Camelot to join her. Determined to stop the loving couple, Morgana frames Gwen for the unpardonable crime of enchanting the prince. With destiny once again at stake, Merlin will be compelled to risk everything to save Gwen from a gruesome death.

Camelot's legendary tournament only has one rule: there are no rules whatsoever. Terrifying warriors start to fill the mythical city, Gilli amongst them. Despite his small stature, Gilli is ready to join the action and committed to achieve victory with a secret weapon to help him: magic. When Gilli begins to realize the full extent of his powers, Merlin becomes torn since his new friend is putting his own life in danger as well as threatening the very heart of Camelot with his actions. Will Merlin be able to make Gilli see sense before it's too late?

Sir Leon is nursed back to health by Druids using the Cup of Life after he is the sole survivor of an attack by Cenred's men. It purportedly grants immortality to those who drink from it, and Arthur and Merlin retrieve it from the Druids to prevent it falling into Cenred's hands. However, they, along with Gwaine, are ambushed and the cup stolen. Having killed Cenred, Morgause makes his soldiers drink from it and, under her orders, they successfully attack Camelot. By the time the three young men arrive back at the court, Uther is in chains and Camelot has a new ruler.

The new queen of Camelot proves to be far crueler than Uther, who is made to watch from his dungeon as she slaughters innocents. Guinevere helps Sir Leon escape from prison and joins her brother Elyan, Gaius, Merlin, Arthur and Gwaine, who are hiding in the woods. Soldiers follow but they are saved by Lancelot and a youth of immense strength, Percival. Merlin looks into the waters of Avalon in the phial given him by the Fisher King, which directs him to Excalibur, the sword which will defeat the enemy. The fugitives find a round table, around which they movingly declare allegiance to each other, and Arthur knights all the young men - except, of course, Merlin, though Lancelot, aware of his powers, tells Merlin he is the bravest of them all. The group launches its invasion of Camelot, which gives Gaius his finest hour as the sisters are routed, largely due to Merlin's use of Excalibur. However, he places the sword in a rock until it can be used again.

Morgana takes the dying Morgause to the Isle of the Blessed where she unwittingly tears the veil to the spirit world, unleashing the Dorocha, invisible ghosts that begin to attack the kingdom. At Camelot, where a weakened Uther has given over much of his rule to his scheming brother Agravaine, Merlin senses what is happening but finds his magic inadequate to stop it. As Camelot fills with refugees, Gaius reasons that, as one death opened the portal, only another will close it. Arthur offers himself as the sacrifice and travels to the Isle of the Blessed with Merlin and a group of young knights. Here, after Arthur has finally praised him for his bravery, Merlin appears to have a fatal encounter with one of the Dorocha.

The kingdom is on the brink of collapse, with Morgana poised to strike. As Lancelot races back to Camelot with the dying Merlin, Arthur must continue on his mission to vanquish the Dorocha without them. Though the knights know their journey to the Isle of the Blessed is fraught with danger, none anticipate the sacrifice Arthur intends to make there. Friendship and loyalty are tested to the limit, but in the end, it is the power of love that changes all their lives forever.

The mighty citadel is buzzing in anticipation of the prince's birthday celebrations, but Arthur's old adversary, Odin, promises to ensure this is one party Camelot will never forget, for amongst the assembling guests lurks a deadly assassin, set to strike. As the dust settles, Camelot is left devastated, but just how far is Merlin prepared to go to put things right? Would he really risk everything and reveal his secret?

The mysterious Julius Borden arrives in Camelot with news of a magical secret that threatens to change the landscape of the kingdom forever. He has located the final part of the key to the tomb of Ashkanar, an ancient mausoleum which holds a very special treasure: the last remaining dragon's egg. Merlin is understandably drawn in by Borden's news, and promises to help him, but just who is the enigmatic stranger and, more importantly, what does he really want with the egg?

Arthur faces his first true test as King when he incurs the wrath of the formidable Queen Annis. With the lives of thousands hanging in the balance, Arthur must find the strength to be his own man and become the leader Camelot so desperately needs.

When Merlin falls into Morgana's hands, he becomes a deadly weapon in her fight for supremacy. Using ancient magic, she pits friend against friend to create the perfect assassin. An oblivious Arthur is in great danger. Will anyone notice Merlin's unusual behaviour before he does the king some serious harm?

Following her defeat by Emrys, Morgana joins forces with the wizard Alator and kidnaps Gaius in an effort to find out more about her arch-enemy.

When the menfolk of Guinevere's home village fall victim to a mysterious illness, Gaius sends Merlin, along with Guinevere and the knights, to cure them, but they are side-tracked by a waif-like girl, Lamia. Only Merlin and Guinevere suspect, rightly, that she is a witch, and soon the knights fall under her spell, succumbing to the same sickness. Arthur rides out with Gaius, who has realized the girl's secret, but will they rescue Merlin and Guinevere or can Merlin defeat Lamia without revealing his magic powers?

Learning from Agravaine that Arthur plans to wed Guinevere, Morgana uses necromancy to revive Lancelot, though he has no proper recollection of his past life and, grateful to Morgana, returns to Camelot to seduce the future queen. Guinevere resists, until she is presented with an enchanted bracelet supplied by Morgana for Lancelot to give her. The two meet in secret and are observed by Arthur. Merlin, suspicious of events, convinces Gaius of how the knight was resurrected, but can they stop Arthur from banishing Guinevere and putting Lancelot to death?

The watery ghost of a slain Druid boy haunts Elyan and captures the conscience of the king.

King Arthur's engagement to Princess Mithian presents him something he's never faced before - a perfect match.

As Morgana's forces capture Camelot, Arthur at last comes to see his uncle's treachery. He is reluctant to flee, but Merlin bewitches him into escaping with him, temporarily blocking his memory and dressing him as a peasant. Elyan, Gaius, and Gwaine are taken prisoner, though Morgana is impressed by Gwaine's performance in trials by combat. Separated from Leon and Percival, Merlin and Arthur fall in with smugglers Tristan and Isolde, during which Merlin's spell wears off and the young king can reveal his identity to them. The quartet escapes to Ealdor, Merlin's village, where a wounded Arthur is nursed by Guinevere as Agravaine and his army close in.

Merlin summons the dragon to vanquish the pursuers and uses his magic to kill Agravaine. Hiding in the woods with Guinevere, Tristan and Isolde, Arthur confesses his self-doubt to Merlin and feels he has failed his subjects, but Merlin proves to him that he is worthy to be king by getting him to extract the legendary sword from its stone. In front of Leon and Percival and many other fugitives from the court, Arthur accomplishes the feat, inspiring them to claim back Camelot by entering through the same siege tunnels and freeing the prisoners. Finally, Camelot will be reclaimed after magic and sacrifice, and Guinevere will be proclaimed its queen.

Arthur has been king for three years without incident, but now an expedition north to the land of Ismere, led by Gwaine and Percival, is captured by Morgana and fellow sorcerer Ruadan and set to work in the mines to locate a mystical key. A search party headed by Arthur and Merlin goes after them, coming upon a village whose inhabitants have been massacred, A dying Druid warns Merlin of Arthur's bane - a battle where the young king will be slain. Ruadan's daughter Sefa is a spy working as Guinevere's maid, who warns her father of Arthur's army heading for Ismere through the lands of his ally Queen Annis. The group is thereby ambushed and Merlin and Arthur captured by Mordred, whom Merlin recognizes from the prophecy as being Arthur's killer.

Ruadan rescues his daughter but is fatally wounded in the process. Merlin and Arthur are route-marched to Morgana's wintry hide-out. Mordred shows them some kindness but Merlin, having glimpsed the future, mistrusts him. Merlin and Arthur escape and arrive in the mines below the castle, where they free all the slaves and meet a spirit called the Euchdag, who is herself the Diamar, the "key" which Morgana had been seeking. Mordred saves Arthur from Morgana's attempt to kill him and is rewarded with a knighthood on the return to Camelot. Merlin remains wary despite the Euchdag telling him that Arthur's bane is Arthur himself, not Mordred.

Arthur rescues an old lady accused of sorcery, who gives him a horn with which to summon the spirit world. Despite Gaius's warning, he uses it to briefly meet his father but soon realises his error as Uther terms him weak for his non-political marriage, his practice of knighting ordinary young men on merit, and his lack of cruelty to make his subjects fear him. When Camelot falls prey to poltergeist attacks, with Guinevere the principal victim, it becomes clear that Arthur has unwittingly released his disapproving, vengeful father's spirit on the court. With a potion made by Gaius to make the spirit visible, and with Merlin's help, he sets out to return Uther to the grave, delivering a few home truths before it goes.

With the help of King Odin who blames Arthur for the death of his son Morgana invades the Kingdom of Nemeth in another attempt to take over Camelot. While Odin holds King Rodor of Nemeth hostage Morgana in disguise as a lady in waiting and Princess Mithian seek help from Arthur and his knights

The druid Osgar delivers the judgment of the disir to Arthur.

After visiting her father's grave, Guinevere is captured by Morgana and imprisoned in the infamous Dark Tower haunted by ghostly visions and screams. Merlin and Arthur lead the knights in a rescue mission but get lost in the thick and aptly named Impenetrable Forest. Mab, the fairy queen of this forest, appears to Merlin and shows him how to guide the others through it but tauntingly reveals that one will not return. Arriving at the tower, they save Guinevere but at the cost of a man. None, however, realise that Guinevere is a changed woman thanks to Morgana.

After Arthur is unseated from his horse, his saddle is shown to have been tampered with and stable groom Tyr Seward blamed. Imprisoned, he tells Merlin he saw the true culprit but was told his mother would die if he informed on them. The actual saboteur, Guinevere (enchanted by Morgana) kills Tyr, and Arthur realises too late his innocence. Guinevere then poisons Arthur and casts the blame on Merlin, leaving an imprisoned Merlin to use magic to save himself and the king. To cover herself, Guinevere summons the apothecary from whom Morgana purchased the poison to pass subsequent blame onto Morgana as the would-be assassin, though Merlin knows better.

Earnest young druid Daegal begs Merlin to come to his village and save his dying sister, but the boy leads him into a trap where he's left to die by Morgana. Arthur meanwhile hosts the ruthless Sarrum of Amata, with whom he plans to form an alliance; however, with Merlin deliberately put out of the way, the bewitched Guinevere, appealing to the Sarrum's lust for power, persuades him to assassinate Arthur on the promise of a share of the kingdom. He accepts (though plans on killing her, too). Guilt-ridden, Daegal returns to save Merlin and they make it back to Camelot in time to thwart the assassination, though with several casualties. Merlin is more determined than ever to discover the cause of Guinevere's unnatural actions.

Friendship, love, and loyalty between Arthur, Gwen, and Merlin.

Morgana captures Alator - her former ally who later sided against her - in order to find out the identity of the legendary warlock known to the Druids as Emrys (still unaware this is Merlin). Merlin is approached by a mysterious woman, Finna, who warns him of danger and asks him to meet her at the temple of Erui. Though she is sent by Alator, Gaius suspects a trap and alerts Arthur to send his knights to find her, though she escapes with Merlin. She gives him a box and warns him against Mordred before willingly sacrificing herself to divert the pursuing Morgana. Back at Camelot, Merlin and Gaius open the box, which contains a prophecy of an imminent great battle. Shortly afterwards, a declaration of war by Morgana arrives.

Merlin sees Mordred helping Kara, a wounded Druid with whom he was once in love, but agrees to keep it secret. Later Kara tries to kill Arthur and is sentenced to death, with Arthur turning down Mordred's pleas for mercy. Mordred uses magic to help Kara escape but they are recaptured and Kara is executed. Merlin rightly fears this will make Mordred determined to destroy Arthur; and, sure enough, he flees and goes to see Morgana, revealing to her that Emrys is Merlin.

Merlin finds himself facing the future he's fought so long to avoid.

With Arthur's forces outnumbered, Merlin arrives in his guise as the aged sorcerer to defeat Morgana's army with magic. Arthur kills Mordred but is wounded and rescued by Merlin, who reveals that he is a wizard. Arthur is initially hostile but, advised by Gaius, allows Merlin to take him to the healing lake of Avalon. On the journey, Arthur comes to appreciate everything Merlin has ever done for him and eyes him with a new respect, even softening in his attitude to magic, but Morgana is pursuing them. Can they make it to Avalon ahead of her?

In poisoning a cup, Nimueh's latest scheme jeopardizes a peace accord between two kingdoms, turns father against son, lays a foe low, and sends Arthur on a perilous quest alone.

Arthur faces his first true test as King when he incurs the wrath of the formidable Queen Annis. With the lives of thousands hanging in the balance, Arthur must find the strength to be his own man and become the leader Camelot so desperately needs.

While hunting, Arthur and Merlin see a unicorn. Despite Merlin's protests, Arthur kills the beautiful creature to bring the crass Uther its horn as an ornament. The foolish father and son ignore Gaius's warning that it is bad luck to kill a unicorn, but soon the crops wither and the well water turns to sand. The mystic Anhora, guardian of the unicorns, arrives in Camelot to tell Arthur that his stupidity has cursed the land and he alone can lift that curse. This involves his learning much-needed humility before riding to the Labyrinth of Gedref, where he will be tested. Merlin follows him but, by doing so, places both boys in danger. Unusually, it is Arthur who saves the day with a demonstration of selflessness, ultimately restoring the welfare of Camelot and the unicorn itself.

Arthur is pressured into an arranged marriage with a visiting princess, but there's more to her than mere gracelessness and a taste for frogs.

Arthur rescues an old lady accused of sorcery, who gives him a horn with which to summon the spirit world. Despite Gaius's warning, he uses it to briefly meet his father but soon realises his error as Uther terms him weak for his non-political marriage, his practice of knighting ordinary young men on merit, and his lack of cruelty to make his subjects fear him. When Camelot falls prey to poltergeist attacks, with Guinevere the principal victim, it becomes clear that Arthur has unwittingly released his disapproving, vengeful father's spirit on the court. With a potion made by Gaius to make the spirit visible, and with Merlin's help, he sets out to return Uther to the grave, delivering a few home truths before it goes.

Rulers from the Five Kingdoms converge on Camelot for a peace conference, including King Olaf and his spoilt, over-protected daughter Vivian. King Alined is also present, along with his jester, the wizard Trickler. He is a disruptive force who wants to cause discord between Olaf and Uther to gain ultimate power for himself. To this end he gets Trickler to bewitch Vivian and Arthur so that they fall in love and incur the annoyance of Olaf, who will challenge Arthur to a duel. This time it is down to Guinevere, rather than Merlin, to break the spell.

Realising that his jousting opponents always let him win because he is the royal prince, Arthur pretends to leave court and enters the lists in disguise to win on merit. He stays with Guinevere, who tells him some home truths about his arrogance, and they kiss. Moorish assassin Myror, paid by neighbouring king Odin (whose son challenged a reluctant Arthur to a duel and lost his life) arrives at Camelot to kill the young prince. He enters the joust with a lance hiding a dagger on its tip, but Merlin uses magic to defeat and kill him. Arthur has learned some humility by his experience but tells Guinevere that Uther would oppose any romance between them.

When a mysterious warrior challenges Arthur to a duel, no one could possibly predict the devastating chain of events it sets in motion - least of all the young prince himself. Arthur and Merlin soon find themselves on a strange quest at the behest of the beautiful Morgause. Through her, Arthur discovers a dark secret which threatens to bring about the downfall of the kingdom.

Merlin summons the dragon to vanquish the pursuers and uses his magic to kill Agravaine. Hiding in the woods with Guinevere, Tristan and Isolde, Arthur confesses his self-doubt to Merlin and feels he has failed his subjects, but Merlin proves to him that he is worthy to be king by getting him to extract the legendary sword from its stone. In front of Leon and Percival and many other fugitives from the court, Arthur accomplishes the feat, inspiring them to claim back Camelot by entering through the same siege tunnels and freeing the prisoners. Finally, Camelot will be reclaimed after magic and sacrifice, and Guinevere will be proclaimed its queen.

Hearing that his village is being threatened by a warlord and his gang, Merlin travels home with Gwen, Morgana and Arthur, the latter doing his best to drill the peaceful villagers into a fighting force. Merlin's friend William is dismissive of Arthur's efforts, feeling they constitute egoism, though he ultimately saves the young prince's life. Nonetheless the villagers are inexperienced in martial ways and Merlin must use magic and conjure up a wind-storm to help see off the bandits.