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In Budapest, honeymooners Peter Alison, a mystery writer, and his wife Joan board a train bound for a resort near a small Hungarian town, and are joined in their compartment by Dr. Vitus Werdegast, a Hungarian psychiatrist.  When the young couple falls asleep, Werdegast strokes Joan's hair, but Peter awakens and catches him in the act.  Werdegast explains that eighteen years previously he left his wife to go to war, and has just returned from spending fifteen years in an infamous prison camp.

When they disembark at the same town, the three share a bus, but just as the driver explains how the area had been Hungary's greatest battleground during World War I, the bus crashes and the driver is killed.  On Werdegast's instruction, Peter carries an unconscious Joan to the fortress-like home of Werdergast's Austrian acquaintance, famed architect Hjalmar Poelzig.  Poelzig has built his home on the ruins of Fort Marmorisch, which he once commanded.

After Werdegast successfully operates on Joan, he accuses Poelzig of selling the fort to the Russians, who killed thousands of Hungarians.  Werdegast also accuses Poelzig of stealing his wife while he was in prison, but their conversation is interrupted by the appearance of Peter and Joan.  When a black cat enters the room, Joan's behavior becomes erratic, and Werdegast throws a knife at it and kills it.  He then explains that cats are symbols of evil and, when they die, their evil goes into the nearest entity.  Poelzig, however, counters that black cats are deathless and that Werdegast merely has a phobia.

That night Poelzig takes Werdegast to a dungeon, where Werdegast sees, to his horror, the body of his wife, encased in a glass coffin.  Poelzig explains that he loved her and also Werdegast's daughter, Karen, who died of pneumonia two years after the war.  Werdegast is enraged and attempts to kill Poelzig, but suddenly cowers when a black cat appears.

Later, while Poelzig secretly visits Karen, who is alive and is his wife, Werdegast plots the murder of Poelzig with his manservant, Thamal.  Werdegast realizes that Poelzig plans to keep Joan captive, and the two men compete in a game of chess to determine her fate.  They are temporarily interrupted by the police, who are investigating the bus accident, and Poelzig mollifies them by promising to drive the Alisons to the train that night.  Peter is alarmed, however, when he discovers that the car is out of commission, his gun is missing and the phone is dead.

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When Werdegast loses the chess game, Thamal holds Peter and Joan hostage in separate rooms, but Werdegast secretly reveals to Joan that Thamal is still working for him, and is obeying Poelzig's orders until the time is right to kill him.  Joan is surprised by the appearance of Karen, who believes that her father is dead.  When Poelzig hears Joan tell Karen the truth, he takes Karen into another room and kills her.

Soon, guests arrive for a cult ceremony in which Poelzig, who is the high priest, intends to sacrifice Joan.  When one of the guests faints, there is a commotion during which Werdegast and Thamal help Joan escape downstairs.  Peter tries to escape but is knocked unconscious by one of Poelzig's servants.  After Werdegast finds Karen's body, he and Thamal handcuff Poelzig and hang him by his arms, after which Werdegast proceeds to skin Poelzig alive.  Peter revives and, hearing Joan's horrified screams, comes to her rescue.  When he sees Werdegast trying to help Joan get the door key out of Poelzig's hand, he misinterprets Werdegast's attempt to help and shoots him.  As Peter and Joan escape, Werdegast pulls a lever that sets off explosions destroying Marmorisch.

Outside, Peter and Joan escape unharmed and flag down a car.  Later, on a train headed for Budapest, Peter reads a review of his newest thriller, in which the reviewer discounts the credibility of the story.