Two men with eight-year old daughters are killed in California and Connecticut, with each having a massive amount of blood drained from his body.
The Connecticut girl, Teena, speaks in a strangely adult manner, telling the agents "men from the clouds" came for her father and "wanted to exsanguinate him." When she's subsequently kidnapped from a social service facility, Mulder and Scully journey to North California and are stunned to discover that the other girl, Cindy, is identical to the first. Both families were involved with a Dr. Sally Kendrick, who was fired from a clinic for experimenting with eugenics-the science of improving human genetic characteristics, usually through breeding.
Deep Throat tells Mulder of the Litchfield Project-a top secret '50's eugenics program that created identical little boys named Adam and girls named Eve. At his direction, Mulder and Scully go to a home for the criminally insane where they find Eve 6, a dead ringer for Sally Kendrick. The Adams and Eves had extra chromosomes, she says, giving them heightened strength and intelligence but also causing them to become murderous. "I can top 265," she says ominously of her IQ. "We're very bright, we Eves."
Two of the Eve clones (one of whom is Kendrick) are still at large and pictures of the Eve youths reveal them to be genetic replicas of Cindy and Teena. Mulder and Scully stake out Cindy's house, but another Eve escapes with her. Brought together, the two girls admit they "just knew" of each others existence and each killed her father – their murderous streak having begun at an earlier age then their predecessors. Eve, who was actually trying to help them, is poisoned.
Authorities find the girls, as Mulder and Scully take them back Mulder realizes they're actually the murderers, just before they can poison him and Scully. The girls try to escape but are caught and jailed with Eve 6. In the last scene they have a visitor: Sally Kendrick, Eve 8. "How did you know I'd come for you?" she asks. "We just knew" is the reply.