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Police have been vainly searching the countryside for the knife-wielding Maniac, who has been on a murderous spree. The Maniac's victims are each found with a taunting newspaper clipping attached to their body. After the wealthy uncle of a young scientist is mysteriously murdered, people wonder if the Maniac is responsible.
Prior to his uncle's death, the young scientist in question, Dr. Arthur Hornsby, claimed to have developed a method of living without oxygen for extended periods. To prove his theory, he had himself buried after taking a dose of the serum. Despite his incapacity, the death of his uncle leaves a vast fortune, which is to be divided amongst his family members and servants. In the event that one or more them dies, the inheritance is split among the remaining survivors. Subsequently, members of the family begin to die, one-by-one, and suspicion is cast on the servants, including the "mystic" butler (Bela Lugosi).
At the end, we discover that Dr. Hornsby faked his burial and was using it as a cover for committing the murders. His plan was to kill any other heirs to his uncle's fortune so that he may obtain sole possession. He is eventually discovered and seemingly killed, but in the closing moments of the film, he comes back to life and claims that he will haunt the audience if they reveal the plot twist to anyone.