On an unexplored planet, Sheppard, McKay, Teyla and Ronon hike into a village at dusk. The startled inhabitants insist that they spend the night. Apparently, after darkness falls, a mysterious killer stalks the area, sucking the life from its victims. Sheppard and his team instantly guess what they're dealing with: a Wraith.
Sure enough, a leading villager, Goran, explains that a spaceship crashed near the village ten years ago. Frightened, the villagers tried to kill all the crash survivors, but one survived and began to feed. Two years later, Goran's own son vanished into its clutches.
As morning dawns, Sheppard and his team head into the forest to track down the Wraith, but they're in for a surprise. The trail leads into a cave, where a young female Wraith cowers in a candlelit laboratory. She lives there with a human scientist named Zaddik, who begs the team not to kill his "daughter."
Zaddik claims that he helped in the massacre ten years ago, but when he found a young Wraith child, he couldn't bear to kill her. Instead, he brought her to this cave, where he has hidden her ever since. He has even developed a serum that he believes allows his adopted daughter, Ellia, to survive without feeding on humans. The two love each other, and they dream of a day when Ellia can be accepted in the village.
Sheppard doesn't let down his guard. A Wraith has been killing villagers, and this girl is definitely a Wraith. Still, he brings in Dr. Beckett to test Zaddik's serum. Beckett arrives with an experimental retrovirus that has the potential to remove a Wraith's alien DNA, leaving behind only human genetic material. It's not finished yet, but someday, perhaps Ellia can fulfill her dream of living as a human.
While Ellia remains at the cave, another villager is murdered. Forced to conclude that Ellia was not the only Wraith to survive the crash, Sheppard sets out with Teyla and Ronon to hunt down the second Wraith.
Unfortunately, Ellia harbors more deadly secrets than anyone guesses. Desperate for acceptance, she risks injecting herself with the retrovirus - without Beckett's permission. The untested drug strips away her human side. With Ellia transformed into a mindless monster, Sheppard's team now must confront two killers.