On New Year's Eve, at a university fraternity party, Alana Maxwell is reluctantly convinced to participate in a prank that twists into a traumatic event. She lures Kenny Hampson, a shy and inexperienced pledge, into a bedroom under the pretense of intimate fun. However, upon entering, Kenny is shocked to discover the students have placed a woman's corpse—stolen from the medical school—in the bed. This horrifying discovery leaves Kenny emotionally shattered to the point of being institutionalized, and he is later sent to a psychiatric hospital.
Fast forward three years, when the same students gather again for a New Year's Eve celebration, but this time aboard a moving train. The party includes Alana, the carefree class clown Ed; the prank ringleader Doc; Jackson; Mitchy, who is Alana’s close friend; her boyfriend Mo; and a magician with his female assistant, hired for entertainment. The atmosphere is festive, but underlying tension begins to surface as the night unfolds into chaos.
Before boarding, Ed is mysteriously murdered, and the killer, wearing a Groucho Marx mask, boards unnoticed along with the others. As the train heads into the wilderness, the killer moves among the passengers, who believe him to be Ed. The situation escalates when Jackson is found murdered in the sleeper car, dressed in a lizard costume. When Carne, the train conductor, discovers Jackson's body, the killer has already hidden the corpse and is now impersonating the lizard costume, with brakeman Charlie thinking he’s merely intoxicated.
As the night continues, the killer’s spree grows more violent: Mitchy is slaughtered in her compartment, and later, Doc finds Mo dead amidst the magic show, highlighting the increasing horror. Carne and Charlie soon find themselves uncovering multiple dead bodies, with the killer leaving a trail of carnage that confounds everyone aboard. In a desperate move to stop the slaughter, Carne pulls the train's emergency brake, causing chaos as everyone is thrown off balance. When the train finally halts, the crew evacuates the terrified students, many dismissing the situation as a prank or stunt.
As suspicion falls on Kenny, Alana, fearing for her safety, reveals that she had tried to visit him at the hospital. She learns Kenny is deeply disturbed and might be connected to the murders. Thinking she’s safe, she locks herself in the sleeper car but is tragically decapitated by the killer, who has now disguised himself as Charlie and is in the magician's costume.
Realizing that the magician was not Kenny, she races to inform Carne but instead discovers Kenny in disguise, having murdered Charlie. He is dressed in a wig and makeup, having posed as the magician’s assistant the entire evening. The horrifying truth hits her: Kenny, the once-shy pledge, is the masked killer. In a tense confrontation, Kenny forces Alana into a kiss, reliving the trauma of the prank years earlier, which triggers a mental breakdown. Exploiting her vulnerability, Carne intervenes, beating Kenny with a shovel. Kenny falls backward through the open door of the train and into an icy river as the vehicle speeds away, leaving the haunting memories of the night behind.
