|Dean and Sam, following instructions left by their father, head to Fitchburg, Wisconsin, not knowing what they are looking for. They discover something strange almost immediately when it is revealed that several children have gotten seriously sick and no-one knows why. Posing as CDC doctors, they meet Dr Hydecker who explains that the children seem to be immune to antibiotics. They also find out that it seems to be spreading from one sibling to another.
The sneak into the house of the latest victims and discover a strange burned handprint on the window ledge and Dean remembers when they were younger that their father had faced this thing before. He wants them to finish the job. The children are being attacked by a Shtriga - a kind of witch - which their father fought 16 or 17 years earlier. Dean and Sam book into a motel and meet Michael, the son of the motel owner. Doing some more investigation into Shtrigas, they find out that they feed on life force sucked from children. They are invulnerable to all man made weapons unless they're feeding and they disguise themselves as humans when they're not hunting. In all, it doesn't look like this one will be easy.
Dean recalls a patient he saw at the hospital, an old woman who had an inverted cross hanging on her wall. They head back to the hospital at night and try to find the old woman. Dean is scared almost to death when the woman suddenly talks to them - she is blind and was sleeping with her eyes open. The cross had simply fallen down during the day.
Doing some more newspaper research, Sam finally realizes that the Shtriga is actually Dr. Hydecker. When they get back to the motel, young Michael tells them that his brother, Asher, has gotten sick like the other children. Not happy about the idea, but knowing it may be the only chance they get, they ask Michael if he would be willing to play bait for the Shtriga. He say's he's a big brother and would do anything for Asher, so he'll help. Sam's still not happy about the plan and demands to know what Dean is hiding from him.
Reluctantly, Dean explains that when they were children themselves, John left them in a motel room in Fort Douglas, Wisconsin, while he went off hunting the Shtriga. After three days, Dean left little Sam alone, asleep, and went off to play video games. When he got back, Sam was being attacked by the Shtriga, just as John got back. They chased off the Shtriga, but Dean would continue to feel guilt for leaving Sam unprotected forever.
That night, Michael waits in bed, knowing that the Shtriga is coming. As soon as it tries to attack him, Dean and Sam run in and shoot at it. They think they've killed it, until it attacks Sam. Dean shoots it in the head with consecrated iron rounds and it is finally dead. Just to be on the safe side, he empties his clip into it. The next morning, Michael's mom comes back and explains that Asher and the other children have suddenly recovered and everyone is going to be fine.