At her family's rural manor in Germany, Irene von Helldorf celebrates her twenty-first birthday with her father Robert, her suitor Thomas Brandt, and Thomas's two rivals, Walter Brink and Frank Faber. Thomas challenges Walter and Frank to each spend a night in a mysterious locked blue room in which several murders had occurred years before, each at exactly one hour past midnight. The first victim was Robert's sister, followed by his best friend and, later, a detective who attempted to solve their deaths. Thomas sleeps there on the first night but disappears exactly at 1:00 a.m. Irene explores the blue room alone the following day, but is attacked by an unseen assailant and loses consciousness, regaining it with no memory of her attacker's identity. Frank agrees to spend the night in the blue room after Robert retires for the evening. Frank begins playing a piano in the room shortly after midnight to signal to the others that he is still alive. Unable to sleep, Irene i...