Sheppard and his team are exploring the planet Thenora, which has very recently been ravaged by a Wraith culling. Buildings are still in flames. They search the nearby woods but find no survivors, and recognize that the Wraith attacks are becoming much more severe. Because all the Wraith woke up at the same time, they are suffering from something of a food shortage -- which is bad news for the Pegasus Galaxy's human population.

Dr. McKay trades barbs with Lieutenant Laura Cadman, a new member of the Atlantis security team assigned to Rodney and Dr. Beckett during the recon. She is no-nonsense but playful, and teases Rodney that he should pick flowers for his impending date with a woman named Katie Brown.

Their banter is interrupted by the omninous sound of a Wraith Dart in the distance. The expedition members race back toward the Stargate, hoping to avoid detection. (Lieutenant Colonel Sheppard makes a cryptic remark about Atlantis's cover being blown if they are spotted, perhaps hinting at the resolution of "The Siege.")

After an escape back to Atlantis, the episode's true story is unveiled: A run-in with Wraith technology has left Lieutenant Cadman's consciousness inside McKay's head! The two must share space while McKay and Zelenka look for a way to power the Wraith device and somehow put things right. Meanwhile, an impatient Cadman insists on being heard (played out on screen in some comical voiceovers) when Rodney tries to ignore her, and Dr. Weir and Sheppard consider the possibility that McKay may be going insane.