Irene von Helldorf celebrates her twenty-first birthday with her father Robert and her three suitors, Captain Walter Brink, Frank Faber and Thomas Brandt. At Tommy's urging, Robert reveals the dark past of the castle's guest room, also known as the Blue Room. Three people, including Robert's sister, his best friend and a detective, were mysteriously murdered in the room, each on a different night. As the door locks from within and there are no other entrances, the murderer remains unknown. In an effort to prove his courage and gain Irene's love, Tommy challenges the other suitors to stay in the room for one night. He takes the first night and by morning, has disappeared and is presumed dead, although Robert refuses to call the police. Irene then is attacked by a stranger in the Blue Room, but when she faints, she is found alone. At midnight, the butler, Paul, signals to someone with a flashlight from the castle. The next evening, Frank goes to sleep in the room as agreed. He defiantly plays the piano, but at one in the morning, a shot rings out and the piano suddenly stops. He is found dead slumped over the piano, with his unfired gun at his side. After the butler calls the police, Commissioner Forster arrives and interrogates everyone. Forster suspects Robert because of his suspicious behavior, and Robert reveals that the stranger who "attacked" Irene is actually his brother and Irene's father, who had disappeared years earlier but has returned because he was destitute. This explained, Walter and Forster arrange a set-up to capture the murderer. Walter spends the night in the room, but at one in the morning, a shot rings out. The bullet hits a dummy of Walter, and he sees an arm holding a gun retreating from a secret panel in the wall. Walter chases the suspect while Forster follows. They are led into passageways underneath the castle, and finally capture the murderer.