Ningning, a 24-year-old living alone, notices her roommate Tang Li acting suspiciously friendly on move-out day, leaving a chilling warning: "Remember to deadbolt tonight." Discovering her spare key missing, Ningning silently replaces the locks and installs hidden cameras. Three days later at midnight, Tang Li returns with the stolen key. Thwarted, her harassment escalates: leave-behind threat notes, bloody warnings at the door, and a spy camera found hidden under the TV stand—revealing her 18-month roommate relationship was a calculated hunting plot.
Even after her arrest, Tang Li sneers that something remains inside. Ningning dismantles her smart speaker to find a modified listening bug streaming her living room audio. The true mastermind emerges: "The Cat," a former electrician who exploited corrupted property managers to operate a voyeuristic blackmail ring. The syndicate leaks edited videos to Ningning's company group chat, extorts her mother, and break in to leave a terrifying threat using a family photo. Finally, "The Cat" takes a security guard hostage, forcing Ningning into an isolated generator room. Working in tandem with the police, Ningning traps and arrests him.
The illicit spy network is dismantled, and all accounts are frozen. Refusing to hide in shame, Ningning publicly clears her name at work, rejects Tang Li's insincere apology, and moves into a secure new apartment—locking her spare key safely inside a code box, victorious over fear.
