Mulder and Scully investigate when a man named Joel Simmons is found dead in his backyard, his body drained of blood, his mouth disfigured. Mulder at first links the incident to mysterious cattle mutilations that have occurred in the United States since 1967.
     The agents interview Teena Simmons, Joel's eight-year-old daughter, who was at home when the killings took place. Teena describes how "men from the clouds" were after her father. Mulder is somewhat mystified, however, when Teena tells him the men wanted to "exsanguinate" her father, a word far too complex for a little girl to know the meaning of.
     Shortly thereafter, the agents receive word of another, almost identical murder that took place in San Francisco, in which a man named Doug Reardon was found dead in his backyard, also drained of blood and disfigured. The agents are stunned when they meet Reardon's eight-year-old daughter, Cindy. She and Teena are identical in appearance. Teena, meanwhile, disappears from the foster home where she is temporarily housed, apparently the victim of a kidnapping.
     Scully investigates when it is discovered that Mrs. Reardon had in-vitro fertilization at a clinic in San Francisco. A physician named Sally Kendrick supervised both the Simmons' and Reardons' IVF program. Kendrick, it is revealed, was fired from her position after she was discovered tampering with the genetic material of fertilized ova in the lab prior to implant.
     Deep Throat contacts Mulder. He reveals the existence of a top secret program once run by the United States government called the "Litchfield Experiment," in which a group of genetically controlled children were raised and monitored. The boys were dubbed "Adam," the girls "Eve."
     Mulder and Scully track Eve Six to a mental institution. To their astonishment, she looks exactly like Sally Kendrick. Eve admits that two other Eves besides herself are still alive. The agents realize that Sally Kendrick was continuing the Litchfield experiment at the clinic in San Francisco where she cloned herself into Teena, Cindy and six other little girls.
     One of the other Eves kidnaps Cindy. She takes the girl to a motel and introduces her to Teena. Both little girls knew of the other's existence, and they murdered their own fathers. Eve is convinced she can raise the girls so they will not suffer from the mental disorders that drove the other Eves mad. Despite her conviction, she realizes too late that the little girls, for no apparent reason, poisoned her drink.
     Mulder and Scully are tipped that the girls can be found at the motel. When they arrive, they find Eve dead. The girls claim the two remaining Eves conspired to kill them. The agents fall for the story, never suspecting the girls played any role in the murder.
     Mulder and Scully transport the girls in their car. They pull off the road at a diner so the girls can use the rest room. Mulder orders four sodas, and when he is not looking, Cindy poisons two of the drinks. Mulder realizes the drinks have been tampered with and stops Scully from ingesting her soda just in time. The girls, however, realize their scheme has been discovered. They play cat-and-mouse with the agents in the diner's parking lot, but Mulder gains the upper hand and arrests them.