Bill Anderson, infamously known as "The Gemini Killer," is a deranged serial murderer grappling with multiple personality disorder. In a chilling scene, he tortures his captive, Allison Perry, in his grim basement using a blowtorch, showcasing his sadistic tendencies.
Meanwhile, Kelly Owen is growing increasingly concerned for her husband, Craig Owen, a successful musician. After requesting him to pick up champagne, she soon finds herself in a harrowing ordeal when Bill abducts Craig in a van that advertises a children's entertainer named "Baylee the Clown."
Craig awakens in a horrifying predicament, bound to a school desk in Bill's basement. Embracing his Baylee persona, Bill begins to inflict merciless pain on Craig, all while mischievously referring to him as Bill. Bill not only adopts the guise of a police officer, believing he is apprehending Craig for the Gemini killings, but also pretends to be an interrogating detective. This character ruthlessly shatters Craig's front teeth with his handgun and forces him to consume them, expanding the grotesque nature of his torment.
Consumed by worry as the hours pass without any sign of her husband, Kelly drives to the liquor store to inquire with Lauren, the cashier, about Craig's whereabouts. When she calls the police, their response is disheartening; they inform her that there’s a mandatory waiting period of 24 hours before they can initiate an investigation. In an attempt to find some solace, Kelly reaches out to her best friend Bianca, who arrives to spend the night supporting and conversing with her.
During his time in captivity, Craig realizes that he has been ensnared by the Gemini and that Bill is projecting his murderous identity onto him. In a desperate bid for freedom, Craig engages Bill in role-playing scenarios. During one chilling exchange, Bill, appearing as a San Quentin convict, shares a harrowing tale of childhood abuse suffered alongside his sister Marybeth. Trying to relate, Craig divulges that his wife also endured abuse as a foster child, but this revelation only fuels Bill's rage, leading him to mangle three of Craig's fingers with wire cutters.
In a twisted form of care, Bill assumes the role of Dr. Jack Goodman, painfully stitching up Craig’s injured hand and recounting another tale from his past, detailing his experiences in a foster home with his sister. Transitioning once again, Bill becomes a lawyer, ostensibly representing Craig prior to his execution. Craig desperately attempts to connect with Bill by recounting a personal story about losing his identity as a musician, but this only meets with Bill's indifference.
As the tension rises, Craig boldly tries to manipulate Bill using the story of Marybeth to evoke guilt, while Bill temporarily embodies his father, a welder, during a visit on Death Row. However, the confrontation escalates when Bill momentarily strangles Craig with a phone cord, reflecting his unstable nature.
In a dramatic turn, Bill takes on the persona of his mother. Craig, sensing an opportunity, appeals to this personality for assistance in escaping, leading to a dramatic moment where Bill clandestinely hands Craig a metal nail file. The twists continue as Bill later masquerades as a prison guard. Just then, deliveryman Andre arrives with Craig's last meal—a pizza—which the pair consumes together. Craig inquires about Bill's infamous Gemini tattoo, to which Bill chillingly replies that he has a twin.
Seizing an opportunity, Craig uses the nail file to free himself from his restraints while Bill momentarily steps out. However, Bill soon discovers Craig’s escape attempt and swiftly recaptures him. Their struggle intensifies as Bill dons the guise of Father Fitzgerald, a Catholic priest. When Craig inadvertently offends him, Bill retaliates by slicing Craig's face with the nail file.
Bill then embodies the executioner, adorned with the Gemini symbol, and despite Craig's desperate pleas, he employs the blowtorch against Craig's neck until the unthinkable occurs—Bill decapitates Craig.
After the horrors of the night, Bill appears at Kelly's door in his police uniform. With an ominous tone, he informs her, “It's done.” Kelly, whose underlying motives begin to surface, hints at a desire for Bill to eliminate Bianca, who remains obliviously asleep on her couch.
In a plot twist revealed through a retrospective view, it becomes evident that Kelly has surreptitiously deduced her twin brother's identity as The Gemini Killer, orchestrating Bill’s gruesome plan to target Craig as retaliation for his infidelity with Bianca.
