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.

Daniel and Vala travel to Atlantis in search of Merlin's anti-Ori weapon, while Sam, and Mitchell join forces with Dr. McKay to lock out the Ori supergate.