Dr. Alfred Morris is curious about the effects of an ancient nerve gas, used by the Mayans during rituals of human dissection to appease their gods. He takes medical student Ted Allison under his wing to assist him with his experiments in using the gas on modern animals. However, despite Ted's enthusiasm for the success of their effort to revive Morris's dead monkey Choco (who was earlier exposed to the gas and died) by using a fluid from the heart of another creature, Ted also has on his mind his girlfriend Isabel Lewis, of whom Morris has also become enamored.
Later, on the night of the duo's first experiment, Ted brings Isabel to Morris's house, where Morris notices Isabel's discomfort about her relationship with Ted. He confers with Isabel, saying that she does indeed need a man more involved with her love of music, secretly meaning himself. Isabel, however, is afraid of hurting Ted's feelings and getting him to understand what she wants, but Morris promises to take care of the situation himself. Unbeknownst to Isabel, Morris's evil plan involves destroying Ted by exposing him to the lethal Mayan gas the next day, and in effect, making him a mindless ghoul who, like Choco, must continually rely on the fluid of human hearts to survive (obtained by performing cardioectomies on freshly dead persons). This sets Ted and Morris on a grave-robbing spree through several towns where Isabel is also performing on her tour. Morris tries to get Ted to return home, but Ted is committed to being with Isabel whenever possible, whenever he is not in his unknowing ghoulish state. But, when Ted does become a ghoul again, Morris once more uses him to try to kill off the one person Isabel truly seems to love—Eric Iverson, her partner and pianist. Although his attempt is unsuccessful, Ted is able to obtain another heart, keeping himself alive.
Eventually the police, aided by ace reporter "Scoop" McClure (Robert Armstrong), realize that the mysterious "ghoul"-style killings are on the same route of Isabel's tour. McClure tries to set a trap in Scranton, the last city of Isabel's tour, by making it seem to the public that he is someone else who has recently died, and, by waiting in a coffin for the ghoul, nearly captures Ted and Morris once they arrive to perform another cardioectomy. However, Morris distracts McClure as Ted comes into view and kills him.
With Isabel back home, the police attempt to question her about why the killings were made in the same cities she performed in, but even though she claims to know nothing, she thinks for a moment how Ted and Dr. Morris are the only people associated with her that also have a knowledge of how to perform cardioectomies. She later performs for her home crowd, and Morris, in a last attempt to get Isabel for himself, sends Ted to kill "First Eric, then myself," as he constantly repeats under his spell. However, before Ted becomes a ghoul, he is able to write a letter to Isabel that explains what happened to him and who did all of the killings. Plus, he exposes Morris to the gas just before he reverts to his ghoul state, and leaves to fulfill Morris's final bidding. Upon entering the stage where Isabel is, he is promptly dispatched by detectives, just as he is about to shoot Eric, who read the note to Isabel that Ted left in his hands. She and Eric hold each other, knowing that Ted always intended the best for them, and that Morris was behind all of the trouble with which Ted and many others were involved.
Morris, meanwhile, being drained of nearly all life by the gas, almost succeeds in getting fluid from another heart for himself, but fatally collapses by the grave he is digging up. In the end, words that Ted said earlier to Morris are repeated: "It's all over, Doctor. There's nothing left of it now but you, and me, and... death!"