A young married woman named Rei lives peacefully with her husband, her boss at work, and her brother-in-law, Mamoru. One day, while her husband is away on a business trip, Mamoru reveals the secret of a basement room. Curious, Rei heads down to investigate—only to discover it's actually a trap set by her brother-in-law, who has long planned to make her his plaything. The moment she enters, she's stripped of her freedom and told, "From today, Onee-san, you'll be living here." From that point on, Rei is imprisoned in the basement as her brother-in-law's captive, subjected to relentless abuse and humiliation in a horrifying ordeal.