Duet

"Duet" is the fourth episode of the second season of Stargate Atlantis.

Plot
While finishing up investigating a planet leveled by a Wraith attack, Dr. McKay and security team member Lt. Laura Cadman are scooped up into a Wraith dart. However before the Dart can escape through the stargate, Sheppard and the rest of the team shoot it down to prevent the Wraith from learning that the Atlanteans are alive and well. Sheppard reports back to Atlantis and has Dr. Zelenka gate over to inspect the wreckage.

After inspecting the wreck, Dr. Zelenka discovers that the Dart's transporter equipment contains the signatures of two life-forms, but only enough power to rematerialize one of them. He forces Sheppard to choose a life form not knowing which is which, then 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.

Cadman insists that she has to be the one to go, partly because she is convinced that McKay will never do so, and partly because it keeps getting harder and harder to hold on. She starts to write a letter home, and requests that McKay deliver it himself, and explain what happened, the next time he goes to Earth. This gives him an idea: interface a gate control module with the dart to build a stable system. Just before Zelenka is to activate the beam, Cadman requests him to wait, and walks over and kisses Dr. Beckett passionately "just in case this doesn't work", an act which McKay finds quite repulsive, telling Zelenka "ugh, just hit it." After de- and rematerialization, both collapse, and are taken to the infirmary.

While they recuperate, Sheppard gets Dr. Weir's approval to include Ronon in his team.

When McKay wakes up, he hears Cadman's voice asking him if he is feeling better. He looks at the empty bed at his left, and fears that the separation did not work, in reality, she is in the bed on his right side. He tells her that he feels a little less crowded, a feeling she shares, pleased to have her own body back. Cadman is then congratulated by Sheppard, for surviving what would probably have been his worst nightmare.

At the very end, we see Dr. Beckett holding Cadman's hand in his, while smiling in a way that indicates that he shares at least some of what she feels for him.