During a lavish gathering at the mansion of the opulent millionaire Roland Voight, a sex worker named Joey stumbles upon a mechanical puzzle box that Voight insists she solve. After successfully figuring it out, she is quickly met with a deadly surprise—a blade springs from the box and fatally wounds her, subsequently unleashing a portal from which chains emerge and brutally dismember her.
Fast forward six years, we meet Riley, a recovering addict who lives with her brother Matt, his boyfriend Colin, and their roommate Nora. Encouraged by her boyfriend Trevor, Riley agrees to break into an abandoned storage facility, where they discover the mysterious puzzle box. After a confrontation with Matt leads to her storming out, Riley finds herself in an empty park where she solves the box but narrowly escapes injury. This triggers the appearance of the Cenobites, a sinister group of mutilated beings who demand she make another choice for sacrifice.
Matt finds Riley unconscious and tries to rouse her, only to inadvertently slice himself on the box's blade, prompting his mysterious disappearance. Grieving and blaming the box for Matt's fate, Riley, along with Trevor, seeks out Serena Menaker, Voight’s former lawyer, who reveals the box’s dark history. A reckless attempt by Menaker to seize the box results in her getting cut and falling victim to the Cenobites.
Determined to uncover the truth, Riley ventures into Voight's deserted mansion where she discovers his journals. They reveal that each configuration of the box has dire consequences—requiring a victim to be marked for the Cenobites to take, and each completion promises a "gift" from Leviathan, the arbiter of Hell. Riley is horrified to see an apparition of Matt, discovering he has been flayed in the process.
As Riley explains her findings to Colin, the still-living but mutilated Voight unexpectedly attacks, leading to Nora’s death by the Cenobites. In their escape attempt, the group crashes their van and finds themselves back at the mansion. Confronted by the Priest, the leader of the Cenobites, Riley faces a horrific ultimatum. She is enticed with a chance to resurrect Matt if she submits to sacrificing two more individuals. After a tense struggle, Trevor is injured by the Chatterer, one of the Cenobites, and Riley is tragically cut by the blade herself.
Riley works to solve the subsequent configuration and manages to rend the Chatterer to pieces, but as events unfold, the true treachery reveals itself. It’s discovered that Trevor has been in collusion with Voight to provide victims for the box. In a desperate attempt to ensnare the last Cenobite, Asphyx, Riley and Colin are thwarted when Voight re-emerges, stabbing Colin with the box himself.
Voight then shares his harrowing story of seeking extreme pleasure after completing his sacrificial tasks, only to be relegated to a life of pain from the contraption he bears. He completes the final configuration, seeking liberation from his torment, but summoning Leviathan heftily confirms his tragic fate.
As sacrifices come to a conclusion, the Cenobites present Riley with an offer to resurrect Matt. Recognizing the inherent twisted nature of such "gifts," she chooses instead to embrace her grief, denouncing the bargain. By opting to live with her burden, she is given the gift of "lament." As the Cenobites disappear and the box returns to its cube shape, Riley and Colin walk away from the mansion, leaving her to ponder if she truly made the right decision. The gruesome fate of Voight awaits him, as he transforms into a new Cenobite within Leviathan’s grasp.
