Carter and Teal'c go in search of the Asgard to try and save Colonel O'Neill, but discover that the Asgard's enemy has returned with a vengeance. The System Lords seek an alliance with Earth against a common enemy.
While the Goa'uld threaten to attack Earth, SG-1 and the Asgard make a last, desperate stand against the Replicators, which have captured Major Carter and invaded the last outpost of the Asgard civilization.
The S.G.C. is put under quarantine after a mysterious infection leaves a Russian officer in the infirmary – but the disease may not be a disease at all.
The impending wedding of his son is the least of Teal'c's worries when the Hak'tyl plan an uprising against the Goa'uld Moloc, driving a wedge between Teal'c and Ishta.
The Stargate is stolen, leading SG-1 to discover that the Trust has taken control of an advanced ship in orbit. Teal'c investigates a series of wide-spread Jaffa deaths.
A duplicate of Colonel Carter seeks help from Stargate Command in defeating the Replicator Fifth, who she claims has found a way to counter the Asgard's new weapon.
The Replicators begin a systematic attack of the Goa'uld, forcing Baal to come to Earth for help. The Jaffa resistance risk their entire movement in an attempt to retake a holy city in Baal's domain.
The fate of the galaxy hangs in the balance as Sam and Jacob search for the only weapon capable of stopping the Replicator onslaught. O'Neill leads a defense of the S.G.C., while Daniel squares off against Replicator Carter.
Daniel must once again decide whether to risk death or ascend when Oma Desala gives him a second chance. O'Neill and Carter struggle with their respective personal relationships.
With the timeline changed by SG-1's actions in the distant past, an alternate version of SG-1 must use the newly-discovered time ship to set things right.
