Reporter Lee Taylor watches as three men visit the morgue to examine the sixth body in a series of "moon murders," strange murders involving cannibalism that always take place under the full moon.  One of the men, Dr. Xavier, pronounces the murders to be the result of a fixation.  Because the murders all occurred in the vicinity of his medical academy, Xavier asks the police for permission to conduct his own investigation and they reluctantly give him forty-eight hours.

The residents of the academy include:  Xavier's daughter Joan; Dr. Wells, a student of cannibalism; Dr. Haines, who might have engaged in cannibalism when he was shipwrecked; Dr. Rowitz, who was shipwrecked with Haines; and Dr. Duke, Rowitz's assistant, and a student of the effects of lunar rays.  Only Wells appears to be beyond suspicion because his missing arm makes it impossible for him to strangle anyone.

Snooping outside the academy, Lee meets Joan.  When he calls on her the next day, she tells him that his news stories have made it impossible for her father to conduct his experiments there.  They all leave for Cliff Manor at Blackstone Shoals, Long Island, and Lee follows them.  During Xavier's first attempt to find the murderer, the lights go out and Rowitz is killed.

Joan volunteers to participate in the second experiment.  This time, all the men except Wells are chained to their chairs.  He secretly attaches synthetic flesh to his arm and face, which enables him to attack Joan, but he is stopped from killing her by Lee, who sets him on fire and pushes him out the window to the cliffs, where he burns to death.