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Dr. Alfred Morris lectures his chemistry students about the possibility that the ancient Mayans discovered a poisonous gas that caused a state of "death in life." After class, Morris tells his prize pupil, medical student Ted Allison, that he has recreated the native formula and asks Ted to work as his assistant. That afternoon, after the two listen to Ted's fiancée, singer Isabel Lewis, perform on the radio, Morris demonstrates the ancient gas's effect on a monkey. He then tells Ted that he has theorized that the Mayans were able to reverse the effects of the gas with the mixture of herbs and heart fluids. After Ted performs an operation on another monkey, Morris revives the first monkey from its zombie-like state. That night, Morris invites Ted and Isabel to his home for dinner, and he tells the singer he knows that she is no longer in love with Ted. The love-struck Morris then tells Isabel that he can arrange for Ted to break off their engagement. The next morning, Morris gases Ted with the Mayan formula, turning the medical student into an obedient zombie. After convincing the helpless Ted that Isabel is no longer interested in him, Morris calls the singer and bids her farewell as she leaves on a concert tour. That night, Morris and Ted rob a grave, and after Ted removes the corpse's heart, Morris returns the medical student to his normal state, with no memory of having been a zombie. Because of the grave robbery, however, Sergeant Macklin leads a police investigation, which is closely followed by reporter Ken McClure. Later, after the experimental monkey has a relapse, Morris realizes that he has only discovered a temporary cure to the gas, and when Ted insists on joining Isabel on tour, the mad chemist agrees to go as well. Backstage after her concert, Ted asks Isabel to marry him, but when she hesitates to answer, he suffers a relapse and again becomes a zombie. Morris then takes Ted to a nearby cemetery in the hope of finding another corpse. There the chemist kills the caretaker and has Ted remove his heart. Later, Isabel confesses to Morris her love for her pianist, Eric Iverson, and after forcing Ted into another relapse, the jealous Morris orders the zombie to kill Eric. He is unable to murder the pianist, however, when Isabel appears. Meanwhile, Ken discovers that "the mad ghoul" has been following Isabel's tour schedule, so he arranges to pretend to be a corpse in the mortuary of the next town on the tour. Morris falls for Ken's trap, but the reporter himself is killed when he fails to see Ted behind him. Returning home, Isabel is questioned by Macklin, who suspects that Eric is the killer. Hearing the description of the scientific and surgical skills of the killer, Isabel suspects Ted of the killings and mutilations, and she goes to Morris for help. Ted walks in on their conversation, and suddenly realizes the truth about himself. After writing a suicide note, Ted lures Morris into the laboratory and exposes the chemist to his own formula. Ted then suffers a relapse and is ordered by Morris to kill Eric, then himself. It is not until Ted leaves, however, that Morris realizes that he has been exposed to the formula. Morris then rushes to the concert hall and begs Ted to help him exhume another body, but the zombie is enveloped by his order to kill Eric and walks on stage during Isabel's concert. Ted himself is killed, however, by the police in attendance, while Morris, still a zombie, dies when he is unable to unearth a fresh grave on his own.