The team is called into battle unexpectedly when the Asgard homeworld is overrun by Replicators -- which have evolved into a startling new form.

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.

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.