The film centers on Chris Vale, Scot Nery, a 22-year-old man who bears the scars of a brutal childhood: he witnessed his mother's rape and murder at the hands of two hired thugs, and he survived a vicious burn to his face that left him grotesquely disfigured. After years spent in an asylum, he makes a harrowing Halloween escape, killing two orderlies who mocked his masked appearance and the memory of his attackers.
In the house now occupied by David Baxter, Derek Osedach and his circle—a girlfriend Shannon, among them—Vale travels there by stealing a costume and a car, after killing a party-goer named Todd Nicholas Daly Clark at a gas station. Everyone at the gathering mistakes Vale for the late party guest, Todd, and the party continues under a tense, uneasy veil as Vale’s presence goes undetected as a killer.
David stages a fake quarrel with a friend to abduct Vale, who is still believed to be Todd, while another friend pretends to be a police officer to aid the deception. However, Vale overpowers the kidnapper and returns with the car to the party, triggering a crackdown when a real officer protocols the scene and orders everyone to disperse. The night devolves into a brutal back-and-forth as Vale re-enters the house, eliminating several guests, and forcibly ties up Shannon, who is wearing a collar that belonged to Vale's mother, a relic tied to the long-buried injuries of his past.
Vale uncovers a concealed hole behind wall boards where his father hid his mother’s corpse before his father’s suicide, a chilling memory that helps explain his relentless fear and rage. In the chaos, one partygoer escapes; David searches for Shannon and finds her captive in the basement. He frees her, but Vale delivers a decisive strike from behind, knocking David unconscious. Shannon, now armed with a gun Vale has dropped, shoots Vale twice, convinced she has ended him.
When the ambulance and police roll in, the fate of the night hangs in the balance. David seems to vanish from sight, though the bodybag he occupies later suggests otherwise. A tense moment unfolds as the officer talks with Shannon: a bag behind the officer’s shoulder sits up, and Shannon seizes the opportunity to grab the officer’s gun and shoot the figure inside. Pulling back the zipper reveals—shockingly—David Baxter lying inside the bag, dead from Shannon’s earlier actions.
The closing image shows Vale hitchhiking on a desolate road. A car pulls up, and the driver assumes he’s in for a wild Halloween detour. The film ends as the vehicle pulls away into the night, leaving his fate ambiguous and the mystery of his crimes unresolved.
