Rei is a young wife who is married to her boss at work and lives a comfortable life with her brother-in-law, Mamoru. One day, while her husband is away on a business trip, Mamoru tells her about the existence of the basement. She heads there curiously, but it is actually a trap set by Mamoru to make her his plaything. As soon as she enters, she is deprived of her freedom of movement and is told, "From today, you will be living here, sister-in-law." From that day on, her miserable life of being confined in the basement as her brother-in-law's toy and being violated in every way possible begins...