The Replicators remain undeterred in their effort to wipe out the Wraith's human food supplies, but the Atlantis team is now armed with the means to track their ships. With time in short supply, Sheppard (Joe Flanigan), Teyla (Rachel Luttrell) and Ronan (Jason Momoa) rush to evacuate those human settlements which lie in the path of the advancing Replicator forces. Meanwhile, both Colonel Caldwell (Mitch Pileggi), commander of the Daedalus and Colonel Ellis (Michael Beach), commander of the Apollo, arrive from Earth. Each is in possession of Asgard based weaponry, ready to confront an armada of Replicator ships. But Dr. McKay (David Hewlett) is not convinced that this is the best offensive and, in partnership with a wraith scientist, struggles to create a program that will effectively shut down the replicators for good. But the team's best laid plans are fraught with hurdles, further fuelled by an unexpected appearance from Larrin (Jill Wagner), the leader of a race of space travelers not unknown to Sheppard.