The team travels to a world and finds out that the people of the planet haven't been culled by the Wraith in a long time. They do learn that a Wraith ship crash landed on their planet 10 years ago and that several Wraith survived the crash. The town folk who found the survivors feared them and killed them outright. However, the town folk know at least one Wraith survived, as people from their town have been killed, slowly over time, since the crash.
The town folk welcome the SGA team's assistance once the team tells them they will hunt down and kill the remaining Wraith.
Now this mission, the team tracks the Wraith to a cave, but find that another of the town folk has been protecting a female Wraith, Ellia, and acting as her father. The father, Zaddik, tells the team that the killings are not by Ellia, but that they must be by another surviving Wraith from the crashed ship. Zaddik also tells the team he has perfected a medicine that allows his daughter, Ellia, not to need to feed on humans.
With this information, the team brings Dr. Beckett to assess Zaddik's claim that his drug can allow a Wraith not to need to feed on a human's life force. Dr. Beckett also tells Zaddik that he has been working on a retrovirus that would remove the Wraith part from a human's physiology. Dr. Beckett goes on to explain that the medication he has been working on has not been perfected yet. He doesn't know if it will work, or what the side effects will be.
Unfortunately, Ellia overhears this and decides to take the untested medication, once the others leave. Ellia, after taking the retrovirus, then disappears. The team then attempts to find her. Later, after eluding the team, she reappears and tells her father that his medicine did not work. She tells him she has had to feed on human life, but believed the town folk and him would believe it was the other Wraith killing them.
The team at this point has already set out to find the other Wraith; the one they believe has been killing the town folk. They track him using Teyla's and Ronon's Wraith perceptive skills. However, Ellia having reverted to a more Wraith form, gets there first and kills the other Wraith, also saving McKay life indirectly.
Zaddik, also searching, finds his daughter, but she is too far gone to recognize him and mortally wounds him. The team comes upon him, wounded, along with the town folk, and learn that Zaddik was the younger son of one of the town folk. Zaddik was the one who had originally found Ellia, and who felt he had to raise her in secret.
He raised her, but then realized she would hunger for human life. He worked on a medication that would prevent her from needing to feed on humans, but before he could perfect his medication for her, he had to give her years of his life in her feeding process.
Unfortunately, Teyla, while searching for Ellia, is attacked and knocked out. Sheppard and Ronon find Ellia and have to kill her. But not after Ellia started to feed on Sheppard.