Dean reveals to Sam the 'big secret' John told him in 2.01 - that Dean must kill Sam if Sam becomes evil.
Sam is enraged, and tries to convince Dean that they have to track down the rest of the Special Children to find out more about what's going on. Dean is reluctant, however, and is more inclined to go to ground instead. He begs Sam to lay low with him for a while, but later on Sam sneaks out of their motel room, steals a car and leaves alone.
Sam shows up at the Roadhouse looking for
Jem, who isn't answering her phone. Ellen tells him that she is hunting up by Yosemite and out of phone range. Sam instead tells Ellen what is going on and she helps him track down a list of children whose mothers died in nursery fires in 1983 - Sam, Max Miller, Andy Gallagher and Scott Carey. Scott's murder by an unknown assailant formed the opening sequence of the episode. Sam decides to investigate.
While staying at the town of Scott's murder, Sam meets Ava Wilson, who has tracked him down after having a vision of Sam's own death (which we have seen her experience earlier in the episode). Ava reveals she also had a premonition of Scott's death. While they are talking in the motel room, Gordon fires upon them from a rooftop opposite.
Dean, who has tracked Sam down with Ellen's help, attacks Gordon in order to stop him, but Gordon wins the fight, knocking Dean unconscious and abducting him.
Holding Dean hostage, Gordon then uses him as bait to lure Sam into an abandoned house he has booby-trapped with grenades. This is the setting and situation of Ava's vision. Gordon reveals to Dean that a demon during an exorcism spoke of a coming war, and named Sam as a soldier in the demon army, and that this is his motivation - he believes Sam is evil (or will undoubtedly become so), and thus needs to be killed. He also cites an 'inside source' at the Roadhouse leading him to Sam's location.
Ava tries to convince Sam to take her with him to the abandoned house, but Sam convinces her to go home instead. Sam manages to avoid the traps, and fights with Gordon, knocking him unconscious and escaping with Dean. As they are leaving the house, Gordon chases them, but is stopped when several police cars pull up (a situation of Sam's engineering), the police arresting him and discovering his weapons stash.
Unable to reach Ava on the phone, Sam and Dean then proceed to Ava's house where they discover her fiance with his throat cut, Ava's engagement ring in a pool of blood on the floor, and traces of sulphur on the window sill. Ava is nowhere to be found.
SuperPretendWiki entry 2x12 Once Upon A Time____