When Scully and Doggett are contacted by a man who claims his wife was murdered shortly after giving birth to an alien baby, Scully digs deeper into reports of alien fertilization on humans, and she begins to question her own pregnancy and its conception.

Mulder and Doggett join forces to investigate the arson attack on a genetics lab where one of the doctors suspected of experimenting with alien impregnation is killed. They encounter Billy Miles, who has been changed into an alien replicant after his recent abduction and is attempting to erase evidence of the tests. When a woman helping Scully out at home is found to be a part of the conspiracy, the agents, along with Monica Reyes and Alex Krycek, know they must protect Scully and her baby from Billy Miles and get her out of the city.