With a new commander of the SGC comes a new leader of SG-1, Lt. Col. Cameron Mitchell, a former fighter pilot who flew an F-302 in the battle over Antarctica. However, he is deterred by the fact that all the other members of SG-1 are leaving for other ventures: Teal'c to manage the Jaffa Nation, Carter to Area 51 for alien R&D, and Dr. Jackson to Atlantis as an Ancient expert. Mitchell endeavors to keep the members of SG-1 for himself, but they are increasingly tough to convince.

Upon finding Merlin's testing grounds, Dr. Jackson, Teal'c, Col. Mitchell, and Vala uncover the hidden treasure including a book that tells the story of the Ancients coming to Earth from another galaxy and a strange alien device.

Daniel and Vala have used the Ancient device to inhabit the bodies of people in the Alterans' home galaxy. They have been taken by a Prior of the Ori to meet the Doci, one who speaks with the Ori, their gods. Daniel surmises that the Ori are what the Alterans who became the Ancients left behind and have ascended in a similar fashion, but without the restrictions.

Vala returns to the SGC upon having been separated from Daniel for a little under an hour. The effects of the bracelets have not diminished even though they have been removed. In order to get the effects reversed, Mitchell, Jackson, and Vala must deal with many people that Vala has used over the years to gain favors.

Vala leads SG-1 to a planet which the Priors have visited with a plan to keep them from worshiping the Ori. Once there she reveals that the symbiote that once held her ruled over this planet. She feels that keeping them in allegiance to her will hinder the spread of Origin to this world.

A Prior of the Ori has established an ever-growing forcefield on a Jaffa-controlled planet called Kalana. A minor Goa'uld named Nerus, who engineered the multiple Gate connections required for the defeat of the Replicators, comes to the SGC with information on the event and offers his help.

Following the death of a Jaffa on Earth and the disappearance of a prominent businessman, SG-1 starts an investigation into what remains of the Trust and into the actions of the Jaffa Nation. They discover that Ba'al is hiding on Earth after losing his forces to the Jaffa and that the Jaffa are secretly attempting his capture.

SG-1 investigates a planet that is said to be the home of the Warriors of Sodan, a legendary tribe of Jaffa said to have not served the Goa'uld in 5,000 years. In an encounter with the warriors, Mitchell deals a deadly blow to one of the Sodan and is captured and taken to their village. There he awaits a battle to the death as punishment for his crime.

SG-1 finds a Goa'uld-Human hybrid in stasis in an abandoned Goa'uld lab and take it back to Earth to study, where they learn it was created using Anubis' DNA.

The Ori unleash a deadly plague on Earth, prompting a powerful ally from SG-1's past to come to their aid.

The plague continues to spread among the US population and the President has quarantined the nation. Teal'c is pleading with the Jaffa High Council to reject Origin, but Gerak arrives, having been transformed by the Ori into a Prior. Cameron and Daniel seek the blood of the Prior that caused the plague in order to cure it, but have once again been captured by the Sodan.

On another planet, Mitchell is placed under arrest for the murder of a scientist.

SG-1 returns early from a mission only to realize that they and Gen. Landry aren't on the same page as far as the the mission they've just completed is concerned. The confusion is quickly resolved when the "real" SG-1 returns at the correct time. Dr. Lam examines them all and concludes that they are identical and thinks they might be clones. Sam however thinks they may have come from a parallel universe as the result of the black hole they created when they destroyed the Ori beachhead. As more SG-1 teams arrive from other alternate universes, one of them includes Dr. Fraiser and another includes the Tok'ra Martouf. It's left to Sam and Martouf to find a way to send them all home.

Leader of the Hak'tyl and their representative on the Jaffa High Council, Ka'lel has fallen in with those who work against a democratic Jaffa nation. Things worsen when she incapacitates and turns a well-spoken brother Jaffa who promoted democracy. Bra'tac comes to Hayes and SG-1 for help in finding a now missing Teal'c and uncovering the shadowy dealings in the Jaffa High Council. On Earth, Mitchell gets an old friend of his into a hospital where alien-based technology is used; hopefully, the advances they have made will be able to cure what ails him.

A contact within the Rand Protectorate comes to Earth with news that the Ori have given his people a superweapon to use against the Caledonian Federation in exchange for their conversion to Origin. SG-1 plans to turn the people from their newfound religion and destroy the superweapon.

While investigating the source of an addictive corn-like crop, SG-1 becomes involved in a firefight. They are forced to fall back to the gate, but as Daniel is dialing both the Stargate and the DHD are beamed away.

SG-1 is tasked to accompany members of the International Oversight Advisory, including Richard Woolsey, on a tour of the Gamma site. The visitors are suitably impressed but everyone is put in danger when a carnivorous insect escapes from the lab. The base is locked down and no one is allowed to travel through the gate. The bug is linked to the Ori - it's appeared on several planets that they've visited - and Daniel thinks it may be their follow-up to the virus. SG-1 and the IOA visitors take refuge in a cave, but Gen. Landrey is left with little choice but to flood the planet with a deadly neurotoxin to destroy the bugs. SG-1 looks for away out before that happens.

Sam manages to activate an Ancient device they found at the Glastonbury site and she and Mitchell are shifted into a phase that makes them invisible to everyone. Daniel soon figures out just what is going on but Dr. Lee's proposed solution sends Daniel to the same plane as Sam and Mitchell. There he is able to translate information explaining that Merlin had created a weapon against the Ori - and provides a gate address where it can be found. Meanwhile, Teal'c travels to a Jaffa planet that has sent out a distress call. The two events are related.

Using the Ancients' communication technology, Vala, who is in the Ori galaxy, contacts the SGC. She tells them that after being sucked into the wormhole she awoke in a bed in a village much like the one she and Daniel had visited when they used the communication stones. The locals think she fell from the sky - a gift from the gods - and befriends the gentle Tomin, a devout follower of the Ori who has a bad leg. Vala has two pieces of information for the SGC: first, she is pregnant though insists that she didn't have sex with anyone; and second, the Ori have been building a fleet of ships and building up its army. Meanwhile, Colonel Chekov advises General Landry that the Russians will not be renewing their working agreement with the United States and will be taking their Stargate.

Based on the information Vala gave them, SG-1 travels to the planet where Merlin may have hidden his secret weapon to defeat ascended beings. There they find an appropriately medieval village and learn that Merlin kept a library there. The locals have never been inside owing to a curse on anyone who dares enter, but SG-1 finds a way in. Once inside, they find a secret chamber that may reveal the location of the weapon. The SGC, meanwhile, has located the Ori's Supergate. Sam's attempt to get control of it is too late, and four Ori ships make their way through. In the ensuing battle, much of Earth's fleet is destroyed, and the future of several SGC members is left in doubt.

As the Ori invasion continues, Vala and Daniel must deal with their leader, Vala's young daughter, who is rapidly aged by the Ori to serve their purposes.

The team goes off-world and ends up in serious trouble. Meanwhile, Landry is faced with having to decide whether or not Vala can be trusted to remain at the SGC.

While Daniel and Vala search for Merlin's anti-Ori weapon in Atlantis' library, Mitchell, McKay and Carter try to to dial the Ori Supergate in the Milky Way.

The SGC is approached by an Al'kesh bomber which lets itself be taken down, so they can capture Ba'al, who offers to exchange the necessary information to find Merlin's weapon for human help to eliminate his clones, which want to kill him. They mistrust his story, as does NID Agent Malcolm Barrett, who, however, fails to take over the investigation. Since the captive might actually be a clone, several missions to different planets retrieve and capture all clones, but which is the real Ba'al, and what are his true intentions?

A mysterious creature targets members of Stargate Command one by one.

Martin Lloyd seeks out SG-1 for assistance when his failed TV show based on the real Stargate program becomes a feature film.

SG-1 is caught in the middle of a war after the Jaffa use a powerful weapon to kill 10,000 Ori followers.

Vala is kidnapped by agents of the Trust who are trying to find an ancient treasure when an accident causes Vala to forget who she is.

Cameron Mitchell must go undercover inside the Lucian Alliance to find the location of the hijacked Odyssey.

During their quest for the Sangraal, SG-1 must join forces with their enemy and face a real-life dragon.

The SG-1 team hadn't counted on a real dragon as a guardian until one flies out, breathing fire. However, Daniel thinks the Sangraal probably is a hologram. They escape, but the dragon follows them outside. Ba'al admits not to know its secret name, while Daniel believes that the name probably holds power over it. While they guess, Cam thinks of luring the dragon to blow it up, but the C4 barely impresses its digestion. Daniel makes it disappear by using Morgan le Fay's name in Ancient, Ganos Lal. They find it had guarded the frozen 'grave' of the realm's arch-magician, Merlin (or rather, Myrrdin). Vala accidentally triggers a mechanism which revives Merlin when she approaches an Ancient repository device. In the meantime, the team is transported to another planet via the obelisk by the gate outside. Sam surmises that the gate and obelisk are programmed to dial automatically every few hours and transport them to a set of planets that are cut off from the rest of the gate system. Unfortunately, they can't dial out to any other planets either while the program is operating. When he comes to, Merlin seems to confuse the team and Ba'al, with Arthurian characters, but once convinced they are on the same side, he goes to work with the Ancient device to create the Sangraal. Merlin dies at the device, wishing them good luck and referring to one last task. Ba'al's stargate programming skills now seem their only chance. Daniel tries the Ancient repository device himself, but is knocked unconscious. Meanwhile, Adria keeps searching for them via the stargate dialing program on different planets- each time a bit quicker- being one planet behind them. While she is searching, the team is once again transported to another planet, minus Merlin's body. Daniel awakens with valuable memories from Merlin and tries to construct the Sangraal using the repository device again, in exhaustingly intense phases, and finds it also gives him telekinetic powers. It also messes with Daniel's mind, having two different personalities and memories present (his own and Merlin's). Before he can finish the work, Sam and Ba'al disengage the dialing device, just as Adria's Ori troops arrive, allowing SG-1 to escape. Adria is able to capture Daniel before he can escape as well.

When an entire people may suffer the fatal wrath of a Prior of the Ori for not converting, General Landry orders Carter to try out an experimental device based on Merlin's technology to transport their whole village. The test works, but before they can activate it on a sufficiently large scale, an Ori force takes the village, wounds Sam and kills villagers near the Stargate before Vala's very eyes. Her husband, Tomin, captures her but claims to not love her. He is ordered, as punishment for being deceived by her, to re-educate her. Sam and Cam are able to activate the functioning part of the device, so their building is cloaked from the Ori, who threaten the village leaders and discover and arrest Teal'c. Even after the remaining villagers bow in submission, the Prior decides to destroy everyone since he couldn't find Sam, which makes Tomin doubt his faith, inform Vala, and confront the Prior. The villagers want to sacrifice Teal'c, but one leader stops them. Having seen the Prior destroy the village, Tomin sends Vala down, where she sees if Carter had managed, just in time, to get the device working...

During a failed experiment, Samantha is transported into a parallel universe where she cloaks Earth to save it from an Ori attack. Getting back to her own universe is not met with sympathy.

Daniel Jackson, who was left behind fighting off Adria while the other SG-1 members escaped through the Stargate, is found working as a Prior: the best in the business, who doesn't need to use any threats to talk people into conversion to Origin. Hoping there is still some of his personality left, the team (plus General Jack O'Neill) kidnaps him. Daniel explains how he temporarily has Merlin's personality and knowledge in himself, and had deliberately allowed Adria to turn him into a Prior, while delaying to finish Merlin's weapon she'd wanted, meanwhile hoping to get an Ori ship for Earth's side. But time is running out, since the team took unexpectedly long to capture him. Still, the generals, Carter, and Vala are most reluctant to go along with releasing him and opening the wormhole he needs to bring the weapon through. Even the White House wants to kill him, or at best, keep him in stasis until he is reverted to his human nature by Merlin's genetic manipulation. But then his plan is wasted- unless the others can be trained to do it in his place (so only they are at risk, not the entire galaxy in case Daniel is Adria's Trojan Horse). After giving them the necessary information, Daniel suddenly frees himself while Richard Woolsey is with him and takes control of a space ship after beaming up Jack too, just after the others have undertaken their mission to Adria's lair, where Daniel, about to be genetically reverted, arrives just in time to tip the balance in their favor.

SG-1 blows up three transport ships in a row; Netan puts a price on their heads. General Landry grants the team members spare time on Earth. Daniel spends it researching in a library, where he meets a bounty hunter who tries to seduce him, then switches to her handgun... but is taken out by a bus. Sam gives a lecture on Air Force technological progress together with Dr. Bill Lee; a sniper nearly misses, but is taken out with an experimental weapon they'd brought along to demonstrate. Teal'c is injured by surprise at a Jaffa camp and successfully traps the killer to try again. Cam goes to his Kansas high school reunion and Vala succeeds in making him bring her along, pretending to be his accountant and partner. His still sexy former flame he'd believed out of his reach, Amy Vandenberg, is initially scared off by Vala, but, divorced and interested, explains she still has a secret crush on him. Alien bounty hunter Odai Ventrell assumes numerous classmates' appearances to kill him. Vala's failed rescue gets them both captured publicly, so he threatens to kill off reunion guests unless the other SG-1 members take their place, but they trick him using Chimera technology; they hope Netan's failure to eliminate them will rekindle the power struggle and Odai reacts surprisingly.

When SG-1, minus Carter, investigates a previously unexplored planet expecting to find themselves in a pyramid, they are astounded to find themselves in a museum. Finding a party occurring in the lobby, they decide to leave and make first contact later, but they find that the DHD is a replica and thus incapable of getting them home. They are discovered and believed to be rebels and are forced to take hostages to buy time for their escape.

Teal'c and Bra'tac are among the numerous victims -though only wounded- of several bomb attacks killing 32 men at a summit in the Jaffa settlement Dar Eshkalon. As soon as physically able, Teal'c swears to avenge them personally, suspecting the honor-less, ambitious warrior Arkad; General Landry refuses to sanction a killing mission without solid proof, but cannot stop Teal'c from going it alone. Bra'tac explains Arkad's sinister past to Daniel, including the possibility of murdering Teal'c's mother. Information from the resistance on another planet indicates Arkad is in league with the Ori and plans to attack Earth. Arkad comes to meet General Landry, offering to help defend Earth against the Ori, denying any part in the bombings, while defending Origin, incriminating a Jaffa sect. SG-1 is now ordered to find and stop at all cost Teal'c, who is using torture. But Teal'c, committed to killing Arkad, reaches his place, but is wounded, captured, and made to duel Arkad after it is confirmed he had killed Teal'c's parents.

Stargate Command gets a video message from Vala's extremely unreliable father, Jacek, on the planet Robak; he claims to know about the late Arkad's plans to attack and blow up Earth, even about the several cloaked, Naquadah-laden cargo ships in orbit, and offers details in exchange for sanctuary. General Landry asks Dr. Lam to help him resume contact with her mother, Kim, his ex-wife. Even after the cargoes are found and blown up, Vala abhors her dad's entry. The general asks her to go easy on him, realizing that he, too, neglected his daughter because of his career, even if Jacek's neglect was just criminal. When Cam and Daniel go tell Jacek he shouldn't scam people, he tricks them into talking to Vala for him. Vala goes over just for catharsis, telling him she's through picking up the pieces of his tricks, and he certainly keeps scamming - as Daniel also finds - even with Terak, a Jaffa from an escaped cargo ship, who hopes to sell the Naquadah. When they are apprehended, Jacek promises to help track it and override the rigging of the cargo ship. The Jaffa wait for Jacek and Vala, intending to eliminate them now that Terak is gone, but SG-1 saves them. Since there never was an override, Jacek cheerfully flies away alone in the cargo ship, ignoring that his treachery was anticipated: it's a decoy.

Vala gambles, wins a cargo ship, but is found cheating; then Adria appears, throws everyone out and claims she has a new Ori fleet and an operational intergalactic Stargate. Vala tells her scary daughter how she helped Daniel, after a dream, reading the mysterious clava thessara-symbol's mystery, which led to a single planet address, where Athena's treasure house must be; after a disastrous visit there, the team refused to follow the next interpretation, fearing it's just a trap set up by Adria, so Vala is declared a security risk, grounded and confined to Area 51, but escapes through the Stargate. Adria takes her along as she hears the team still plans to visit the address, by starship. Upon arrival, they are trapped by SG-1, which however finds itself surrounded by lord Ba'al's troops. Ba'al uses the rings to beam up Adria into an electromagnetic cage in his starship. Back on Earth, General Landry worries what damage the two might do together and wants Adria back. Daniel and Sam now show Vala she has been implanted by Galaran technology with fake recollections of her entire adventure 'til the meeting with Adria. Even though she claims that's impossible, Ba'al is confident he can take control over Adria's loyal Ori troops through her- by implanting a symbiote. The team finds Ba'al's Jaffa and clones killed by symbiote poison, except one Jaffa who secretly used tretonin. The team ring-beams from the cloaked Odyssey starship into Ba'al's, shoots him and captures Adria, but back on Earth, she is found to be under the control of his symbiote. Instead of just killing the evil double, they contact the Tok'ra which send Ta'Seem to extract the symbiote and replace it with a Tok'ra one, but Ba'al manages to poison Adria, who has to be put down. Suddenly Adria wakes up, uses mind-power barricading herself in the sick bay with Vala, tells them she needs time to prepare for ascension, as Daniel guesses outside, and succeeds before the team can cut their way in by blow torch. Nobody knows what this will mean for the future, especially as it remains uncertain whether the Ori are still alive. If not, she may have gained all their former powers, and at least one clone of Ba'al still remains...

General Landry accompanies SG-1 on a mission to collect the knowledge of the Asgard. When the Ori show up, however, Sam is forced to evacuate the crew. Landry and SG-1 stay aboard, but are stuck helpless in a time dilation field for decades.

SG-1 investigates a planet that is said to be the home of the Warriors of Sodan, a legendary tribe of Jaffa said to have not served the Goa'uld in 5,000 years. In an encounter with the warriors, Mitchell deals a deadly blow to one of the Sodan and is captured and taken to their village. There he awaits a battle to the death as punishment for his crime.

On another planet, Mitchell is placed under arrest for the murder of a scientist.

Cameron Mitchell must go undercover inside the Lucian Alliance to find the location of the hijacked Odyssey.

Leader of the Hak'tyl and their representative on the Jaffa High Council, Ka'lel has fallen in with those who work against a democratic Jaffa nation. Things worsen when she incapacitates and turns a well-spoken brother Jaffa who promoted democracy. Bra'tac comes to Hayes and SG-1 for help in finding a now missing Teal'c and uncovering the shadowy dealings in the Jaffa High Council. On Earth, Mitchell gets an old friend of his into a hospital where alien-based technology is used; hopefully, the advances they have made will be able to cure what ails him.

SG-1 is tasked to accompany members of the International Oversight Advisory, including Richard Woolsey, on a tour of the Gamma site. The visitors are suitably impressed but everyone is put in danger when a carnivorous insect escapes from the lab. The base is locked down and no one is allowed to travel through the gate. The bug is linked to the Ori - it's appeared on several planets that they've visited - and Daniel thinks it may be their follow-up to the virus. SG-1 and the IOA visitors take refuge in a cave, but Gen. Landrey is left with little choice but to flood the planet with a deadly neurotoxin to destroy the bugs. SG-1 looks for away out before that happens.

With a new commander of the SGC comes a new leader of SG-1, Lt. Col. Cameron Mitchell, a former fighter pilot who flew an F-302 in the battle over Antarctica. However, he is deterred by the fact that all the other members of SG-1 are leaving for other ventures: Teal'c to manage the Jaffa Nation, Carter to Area 51 for alien R&D, and Dr. Jackson to Atlantis as an Ancient expert. Mitchell endeavors to keep the members of SG-1 for himself, but they are increasingly tough to convince.

Following the death of a Jaffa on Earth and the disappearance of a prominent businessman, SG-1 starts an investigation into what remains of the Trust and into the actions of the Jaffa Nation. They discover that Ba'al is hiding on Earth after losing his forces to the Jaffa and that the Jaffa are secretly attempting his capture.

While investigating the source of an addictive corn-like crop, SG-1 becomes involved in a firefight. They are forced to fall back to the gate, but as Daniel is dialing both the Stargate and the DHD are beamed away.

Sam manages to activate an Ancient device they found at the Glastonbury site and she and Mitchell are shifted into a phase that makes them invisible to everyone. Daniel soon figures out just what is going on but Dr. Lee's proposed solution sends Daniel to the same plane as Sam and Mitchell. There he is able to translate information explaining that Merlin had created a weapon against the Ori - and provides a gate address where it can be found. Meanwhile, Teal'c travels to a Jaffa planet that has sent out a distress call. The two events are related.

A mysterious creature targets members of Stargate Command one by one.