Noriko Kago, who has promised to marry her boyfriend, moves to Tokyo due to her father's job transfer and begins attending a new school. One day, she falls ill during class and heads to the infirmary, where she is lured by the school nurse, Sasagi, and taken to an underground facility and imprisoned. As she attempts to escape, even greater dangers close in on her. The intense tension and overwhelming sense of dread heighten the story's dark descent.