Mulder and Scully investigate a number of killings in which the victims had the word "SISTER" carved on their chests and find that serial killers can in fact be passed down through the generations.

BJ Morrow and Lt. Tillman are having a secret affair. When she tells him that she is pregnant, Tillman directs her to the nearest motel where they can talk. Instead she finds herself in a field out of control as she uncovers the long lost body of a murdered FBI agent, Sam Chaney missing since 1942.

Mulder and Scully do some background checking and find that while he was alive, Chaney was investigating a series of murders where the word SISTER was carved on the victims chests and that the killer was never caught. While talking with BJ, Scully finds that she is pregnant with Tillman's baby. Mulder is intrigued into how she found the body but she says that she just doesn't know. While the agents are trying to find out what is carved on Chaney's rib cage, BJ suddenly works it out as saying BROTHER. Then Tillman comes in and says that there has just been another homicide in which the same words were carved. The killings are starting up again.

BJ does her own investigation and finds that a man named Harry Cokely matches the man she has been seeing in her strange dreams and who was also convicted for the killings a long time ago. The agents pay Cokely a visit but find that he is just an old man who is incapable of even standing up, never mind killing someone. Yet BJ wakes up in the night and finds herself covered in blood with the words written on her chest. When she looks in a mirror she sees the reflection of the young Harry Cokely behind her but no one is there.

This leads BJ to suddenly lift up the floorboards in someone's house. When she is stopped from doing so, Mulder continues the find for whatever she was looking for and finds that underneath the floorboards are the remains of Chaney's old partner who also went missing. They also finds that the house they are standing in is the same as Cokely rented 50 years earlier.

Mulder and Scully pay a visit to a woman that Cokely raped a long time ago, Mrs. Thibodeaux and find that she had Cokely's baby years earlier but that she gave it up as she didn't want it turning into the same "monster" as Cokely had. Mulder realizes that the baby she gave up would have now been the same age as BJ. This leads Mulder to believe that BJ is in fact Cokely's grandfather and that the genetic murdering trait has been passed down through the generations. BJ is the one who is continuing the crimes.

Mulder also realizes that she must be finishing what Cokely couldn't, to kill her grandmother so Mulder rushes over there. As BJ looms over her grandmother with a scalpel she says "Someone has to take the blame little sister" but at the last minute she doesn't kill her. Instead she goes after Cokely. She kills him just before Mulder enters when BJ ambushes him and holds the razor to his throat. Scully enters and holds a gun on BJ who drops the razor when Scully says that Cokely is dead. The crimes are over.

As BJ is being held in a psychiatric clinic Scully says that her pregnancy brought on the transformation from normal to insane. Instead her baby is being adopted by Tillman and who knows what the baby will grow up to be......