An away team is investigating a planet, when suddenly a Wraith dart arrives. Major Sheppard and his team are able to shoot it down, but not before McKay and a soldier, Laura Cadman are scooped up. As there is not enough power in the Wraith dart to re-materialize them both, only McKay is taken out. He passes out immediately and awakens on Atlantis. There is something wrong however. Cadman has appeared in his mind. She is able to see and hear what he does and speak to him, driving him mad. After a meeting with psychologist Heightmeyer, Cadman realizes she also can take over McKay's body. While Zelenka works on a solution, major Sheppard tries to convince Ronon Dex to join forces and convince Weir that he is a worthy addition.