Having been savagely bitten by the Wraith girl Ellia, Sheppard hurries back to Atlantis for medical treatment. Both he and Dr. Beckett are shocked, however, when they realize that the wound has vanished. Sheppard's arm is completely free of any trace of the bite.
Beckett starts running tests. While waiting for the results, Sheppard goes on a run with Ronon - and beats him without breaking a sweat. Then Sheppard spars with Teyla, and beats her easily, too. His strangely high spirits overwhelm him and he suddenly kisses a very surprised Teyla - which is when Beckett calls him back to the infirmary.
Sheppard has been infected with the Wraith retrovirus that Ellia was carrying. Beckett hopes that the substance will break down harmlessly in Sheppard's body, but he can't be sure. Weir orders Sheppard to stay in Atlantis and report for check-ups every six hours until they have more definitive answers.
That night, a strange, scaly patch appears on Sheppard's arm. Dr. Beckett confirms what Sheppard himself already fears: The retrovirus is at work, transforming Sheppard into a mindless Wraith. As Weir reluctantly reassigns Sheppard's duties to Col. Caldwell, Sheppard finds himself confined to the infirmary, facing a doomed struggle to preserve his sanity.
Beckett's team devises a risky solution: theoretically, they can inject Sheppard with Iratus bug stem cells programmed with human RNA. The catch is that someone will need to collect live Iratus eggs - and Iratus bugs are lethal killers, just like the Wraith who evolved from them.
Sheppard asks Weir to let him lead this dangerous mission. She refuses. In furious frustration, he slams his fist through a glass window - proving, to his dismay, how much self-control he has already lost.
Leaving him behind, McKay, Teyla, Ronon and Beckett take a team to the Iratus bug homeworld. There, they venture into a cave to find the creatures' nest. The place is swarming with bugs. As Beckett tries to steal an egg, the bugs attack. Two men are killed before the team can escape - without the eggs. A second attempt is unthinkable.
Sheppard's transformation grows worse. In another day, he'll be gone forever. Seeing no hope, his friends struggle to accept defeat - but Sheppard himself might hold the key to his own salvation.