Christopher, a solitary meter maid, stumbles upon an invitation for a Halloween costume party named "Murder Party" while walking down the street. Excited, he ingeniously crafts a knight costume from cardboard and heads to Brooklyn to join the festivities, only to find that the event is a trap set by a group of unhinged art students. Dressed in bizarre costumes, the group includes Paul, who portrays a gothic vampire, Macon as a werewolf, Sky as a zombie cheerleader, Lexi, reminiscent of Pris from Blade Runner, and Bill, embodying a character from The Warriors. Their chilling goal? To commit a murder as part of an art installation to impress Alexander, their rich and menacing patron, with the hopes of securing a lucrative arts grant.
As Christopher attends the party bearing a loaf of pumpkin raisin bread, it isn't long before chaos ensues. Sky, after taking a bite of the bread, reveals her allergy to non-organic raisins, which leads her to collapse after hitting her head—prompting the group to hide her body out of sheer embarrassment. Tension escalates when Alexander makes an entrance, accompanied by his mysterious friend Zycho, who doubles as his drug dealer, and his dog Hellhammer, purportedly part dingo.
The group's dynamics take a darker turn as they begin discussing how to carry out their murder plot, fueled by drugs and alcohol. They plan to stab Christopher simultaneously at the witching hour. In the meantime, while Macon steps out for pizza, Alexander engages in a sensual escapade with Lexi, and later with Paul, much to Macon's dismay upon his return.
Bored with the situation, the group decides to unwind with a game of truth or dare using Sodium Pentothal, or “truth serum.” As they each take their turns revealing trivial secrets, they also attempt to force Alexander to confirm the existence of his promised grant money. Bill, who had remained mostly aloof, discovers that they plot to kick him out simply because his artistic talents surpass theirs.
The tension rises when Macon, frustrated with his unrequited feelings for Lexi, recounts their innocent high school moments, showcasing a discarded popsicle stick as a keepsake, only to be rebuffed by her. As the atmosphere grows increasingly oppressive, Paul decides to involve his professional assistant, igniting an argument within the group. In a moment of desperation, Paul injects Alexander with truth serum, uncovering the shocking truth: Alexander is a fraud posing as a fry cook who intends to kill the group and exploit their art for profit.
In a dramatic twist, chaos erupts as Macon accidentally catches fire. Lexi and the assistant spring into action to extinguish him, but the madness does not end there. Zycho returns with drugs, and under Alexander’s orders, the mayhem escalates with multiple murders taking place in shocking speed. After suffering through burns but remaining alive, Macon utilizes a chainsaw to retaliate against Zycho.
Just when it seems hope is lost, Lexi tries to untie Christopher, but the increasingly deranged Bill decides to eliminate everyone in the room, resulting in Lexi's tragic death. Hellhammer, in a bizarre twist of events, consumes the drugs and turns on Alexander. Bill, now a full force of destruction, hunts down Christopher, who manages to escape amidst the chaos.
The dread culminates at an art student party where Christopher attempts to hide in an art exhibit. Meanwhile, Bill, misled by the promise of a grant from Cicero, spirals further into violence, taking countless lives there. In a final showdown, Christopher seizes a hanging chainsaw, using it against Bill in a desperate act of survival, exclaiming “I JUST WANTED TO PARTY!” Following the gruesome encounter, he calls emergency services while attendees mistakenly perceive the horror as part of an art exhibition.
In the aftermath, Christopher rids himself of his anti-anxiety medication and returns home to a familiar sight: his cat, Sir Lancelot, reclaiming his chair. Plopping down, Christopher turns on the television, still clad in his blood-stained knight costume, a haunting reflection of the night’s dark unfolding.
