The team find an outpost of the Ancients and an alien who claims to be a member of this lost race. Using mind control technology, the alien forces the team to take him back to Atlantis where he immediately tries to take over the city in revenge for being wronged by the Ancients. With the Stargate Atlantis team succumbing to the effects of a virus, Major Shepherd must fight against madness long enough to rescue the city from the alien.