A killer known as The Maniac is on the loose near the grounds of the Rinehart estate, where Arthur Hornsby explains to the elderly Professor Richard Rinehart that his formula will prevent people from dying when buried alive. Later, the gravedigger, who is preparing a grave for Arthur's demonstration, is killed, and Sika, wife of the servant, Degar, tells everyone that death hangs over the house. Late that night, Professor Rinehart is killed in his lab, and when fast-talking reporter Tom Hartley arrives with the police, they find the gravedigger's body as well. As the Rinehart relatives, John and Aunt Sarah, gather to hear the reading of the will, Arthur continues his preparations for the experiment. After a group of professors bury Arthur, Tom, who has called for a taxi, finds the car's driver stabbed to death by The Maniac, and rushes to town to notify the authorities. Meanwhile, John is killed by an unknown assailant when he tries to get the formula to revive Arthur, and the police arrive. The remaining members of the household hold a séance, during which Sika is killed. Degar is accused of the crime, but escapes. Then, Mary is seized by a masked figure, who quickly disappears when she faints. When Mary revives, she sees The Maniac and her screams summon help. Arthur's coffin is dug up and found to be empty, but Degar then leads Arthur through an underground passage into the grave, and all believe his experiment worked. However, Degar explains that actually a trapdoor connected the coffin with a tunnel leading to the house, allowing Arthur to sneak out and commit his crimes while his burial acted as his alibi. Arthur planned his experiment to divert suspicion from himself as he killed members of the Rinehart family. With Arthur caught, the uncaptured Maniac addresses the audience from the next room and warns them that if they reveal his identity, he will haunt them and tear them from limb to limb.