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Ten years after the death of millionaire Cyrus Norman, his attorney, Crosby, and his six remaining relatives, Wally Campbell, Joyce Norman, Fred Blythe, Charlie Wilder, Aunt Susan and Cicily, arrive at his gothic mansion in the eerie bayous near New Orleans to hear the reading of the will.  Although Crosby discovers that the will, which was to be read at midnight, has been opened, he begins the reading at the prescribed hour, but is interrupted as Miss Lu, Cyrus' housekeeper, who believes in spirits, predicts that someone will die before the night ends.  The others are disappointed to learn that Joyce is the sole heir, but because a streak of insanity runs in the family, Cyrus has left a second will in case anything happens to her, thus placing Joyce's life in danger.

More danger looms as Hendricks, a guard at a nearby sanitarium, appears with the news that "The Cat," a homicidal maniac, has escaped.  The news puts everyone's nerves on edge, and as Crosby is about to warn Joyce of the danger, a hand appears from behind a panel and he vanishes.

Later that night, Joyce and Wally follow Cyrus' clues to find a hidden diamond necklace that is later stolen when a hand appears from behind Joyce's bedroom wall.  Because Joyce alone had been present when both Crosby disappeared and the hand appeared, everyone begins to think that she is going insane until they find Crosby's dead body behind a hidden panel.  When a death knell is sounded, Wally becomes suspicious of Hendricks and Lu and goes to investigate.

As Joyce waits alone in the library, a panel slides open, revealing a secret passage.  Entering the passage, Joyce is trapped and chased by The Cat to a hut outside.  Just as The Cat is about to attack, Wally appears and unmasks Charlie as The Cat and Lu shoots the intruder.

As daylight arrives, danger dissipates and Joyce awards the house to Lu, then departs with Wally.