SG-1 returns to Stargate Command with news that Earth has been saved from the Replicators — only to learn that one of the deadly creatures was not destroyed. After crashing into the ocean aboard the ship of Asgard leader Thor, the Replicator bug found its way into a Russian submarine, where it killed the entire crew and continued to rapidly replicate. Col. Jack O'Neill suggests the sub be towed out to sea and nuked, but the Pentagon feels that such an act could cause a serious situation with the Russian government. Instead, O'Neill and Teal'c lead a small, heavily armed team aboard the sub in the hope that the Replicators can be eliminated by sheer firepower. Maj. Samantha Carter is taken to the Asgard's home planet, Cimmeria, which is on the verge of total destruction by the Replicators. Thor believes that Carter's human knowledge might hold the key that will enable the Asgard to defeat the Replicators once and for all.