While investigating a planet leveled by a Wraith attack, McKay and new security team member Lt. Laura Cadman are transported aboard a Wraith Dart. Before the Dart can escape through the stargate, Sheppard and the rest of the team shoot it down. They race to the crash site, but there's no sign of McKay and Cadman.
After inspecting the wreck, Dr. Zelenka announces that the Dart's transporter equipment contains the signatures of two life-forms, but only enough power to rematerialize one of them. He activates the materializer, and McKay appears. The group returns to Atlantis, hoping that McKay will find a new way to power the Dart and retrieve Lt. Cadman. Then, however, McKay claims to hear Cadman's voice - from inside his own head. Somehow, her consciousness has been downloaded into his body along with his own.
While Zelenka works to repair the materializer, McKay and Cadman struggle to co-exist. After Cadman learns to control McKay's body, she can't resist making a few improvements - forcing him to exercise and giving him a woman's perspective on his big date with Dr. Katie Brown. McKay, however, is not even slightly amused.
Meanwhile, Sheppard introduces Ronon Dex to life in Atlantis. He quickly grows to respect the enigmatic warrior's incredible abilities both with weapons and in hand-to-hand combat. Eventually, Sheppard decides that Ronon belongs on the Atlantis team - but he'll have to persuade both Dr. Weir and Ronon himself that this is the right decision.
After Zelenka's latest attempt to repair the materializer fails, McKay and Cadman's situation grows dire - and not just because they're getting on each other's nerves. When they suffer a dangerous seizure, Dr. Beckett informs them that the strain of containing two consciousnesses has become too great for McKay's body. One of them must voluntarily yield complete control to the other - and disappear forever - or they will both be lost.