When a young boy mysteriously dies, Mulder and Scully must try to stop a boy who may be possessed by the devil.

As a boy follows his balloon at a theme park, he is suddenly drawn to the middle of a train track where the balloon stops. He is killed when the train is unable to stop and as the shocked parents look at their dead child, his eight year old brother, Charlie looks as if he really doesn't care.

Mulder is intrigued by a photo of the incident showing what looks like a ghostly figure pulling the balloon when he has the photo enhanced. This leads the agents to meet with the boy's parents who swear that the death was a definite accident. When Scully slips out to look around the house, she sees Charlie's grandmother drawing a swastika on the child's hand, the symbol used to ward off demons.

Scully, later looks over the boy's records and finds a unusually large amount of recurrent illnesses so suspects that it may be a case of Munchausen by Proxy, in other words, child abuse where the parent regularly creates false illnesses in a child. The father, Steve is outraged at the accusation but admits that things have been a little strange since the grandmother came to stay, the grandmother who thinks that her grandson is the devil.

Soon after, Steve, due to Scully's recommendation, agrees to take Charlie to Scully's old social worker but as he is in the garage, he suddenly gets strangled to death when the garage door-opener catches his tie much to the distress of the young boy.

An investigation into the family's house leads to the discovery of a number of dead roosters in the grandmother's bedroom and Mulder finds a strange, holy ash in the garage capable of materializing out of thin air. As this is going on, the grandmother takes Charlie to her room with three members of the Calusari, a group of Romanian holy men, and starts to perform a strange ritual which suddenly arises the image of Charlie cursing the group. When the social worker arrives, they find Charlie on the floor very ill.

Suddenly, the grandmother takes the boy and locks him in the room with her but just as she is about to kill him, he stands over her and drops two roosters on her face which peck her to death. Mulder again finds ash at the scene which prompts him to talk to the Calusari who say that they are trying to cleanse the house of evil.

Charlie finally gets to talk to the social worker but during the interview he suddenly goes beserk claiming that he wasn't the one that killed his grandmother, it was Michael. Maggie, the boy's mother, explains to Mulder and Scully that Michael was his twin who died at birth. His grandmother wanted to perform a ritual there and then to separate the two souls but instead it all went wrong and Michael's spirit was unleashed. Later, Charlie suddenly has a seizure and is rushed into hospital but soon Michael appears and attacks the nurse and asks his mother to take him home, she thinks it is Charlie until he starts acting very weird.

The agents see that the ghost of Michael is leaving so as Mulder and some of the Calusari perform an exorcism on Charlie at the hospital, Scully goes to find Maggie and Michael but is suddenly thrown against the wall by an unseen force and pinned there. Just as Michael is about to stab Scully, the exorcism finishes and the spirit disappears.

As Mulder prepares to leave with Scully, one of the Calusari mentions to him, "Be careful, it knows you" as Mulder notes that the case remains an unsolved mystery.....