The film begins in a hotel room where Emma is ending an extramarital affair with Tom, a lawyer who works at her husband firm. Emma feels deeply guilty and trapped in her cold marriage to Mark, a controlling and wealthy defense attorney. That evening, Mark surprises Emma with an anniversary gift, a heavy steel necklace, and drives her blindfolded to an isolated lakehouse in upstate New York where they used to spend summers together. After a fancy dinner and what seems like a romantic reconciliation, Emma falls asleep. She wakes up the next morning handcuffed by her left wrist to Mark. As Emma stares at him in confusion, Mark tells her he knows about the affair, points a gun to his head, and pulls the trigger. His blood and brain matter splatter across the bed and Emma face. In pure shock, Emma tries to unlock the cuffs, but Mark swallowed the key or threw it away. Emma drags Mark heavy body across the bedroom to find help. She finds Mark phone, but he submerged it in a vase of water before shooting himself. She drags the body downstairs and finds the landline phone cord severed. She reaches the garage and climbs into their SUV, but Mark drained the gas tank to empty. Just as despair sets in, Emma spots a car driving up the snowy road. It is Tom, who received a strange text message sent from Mark phone asking him to come to the lakehouse. Before Tom can free Emma, another vehicle pulls into the driveway. Out steps Bobby Ray, a violent criminal who spent ten years in prison after attacking Emma years earlier, along with his younger brother Jimmy. It turns out Mark promised Bobby Ray a stash of diamonds inside a safe in the lakehouse in exchange for killing Emma. Mark orchestrated this entire day so that Emma would be trapped, murdered, and blamed for his own death. When Tom tries to stall the brothers, Bobby Ray stabs him in the heart and kills him on the porch. Emma retreats into the shadows of the house, dragging Mark corpse through laundry rooms, kitchen cabinets, and into the dark crawlspace. Using tools, golf clubs, and wits, Emma begins picking off her attackers one by one while trying to saw through the steel handcuffs before Bobby Ray cuts her throat.

In the climax, Bobby Ray and Jimmy find Emma hiding inside the house. Bobby Ray discovers the safe inside the master bedroom closet. The safe requires a fingerprint code. Bobby Ray cuts Mark thumb off to open it, but the safe reveals another lock that needs a combination code. The code is the date of Mark and Emma wedding anniversary, which Bobby Ray does not know. Jimmy does not want to murder Emma and tries to convince his brother to leave with whatever they have. However, Bobby Ray is furious and refuses to walk away empty-handed. When Emma offers to open the safe in exchange for her life, she punches the numbers in, but the safe only contains a tiny note from Mark mocking Bobby Ray. The real diamonds are hidden inside the heavy steel necklace locked around Emma neck. Bobby Ray tries to decapitate Emma to take the necklace, but Jimmy steps in to defend her. During the struggle, Bobby Ray accidentally tackles his brother into a glass table, fatally impaling Jimmy on a sharp shard. Enraged by his brother death, Bobby Ray hunts Emma outside onto the frozen lake. Emma lures Bobby Ray out onto the fragile ice. Still handcuffed to Mark dead body, Emma uses the heavy corpse as an anchor. When Bobby Ray tackles her, Emma grabs Mark revolver and shoots through the ice. The ice fractures, plunging Bobby Ray, Emma, and Mark body into the freezing black water below. Under the ice, Bobby Ray tries to drown Emma, but Emma stabs him in the eye with the handcuff chain. Bobby Ray drowns, pulled down into the depths by the weight of Mark body. Emma manages to kick herself free to the surface, breaking through the ice hole and gasping for air on the frozen lake as police sirens echo in the distance. The final shot shows Emma lying on the ice, having beaten Mark cruel death trap through pure resilience.