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

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?