In two-color and based on a play by Howard W. Comstock and Allen C. Miller, a doctor experiments at his academy to find a lunatic murderer.
Lee Taylor (Lee Tracy) reports on the "moon-killer" who has strangled a sixth victim near Dr. Xavier's medical academy; each corpse had an arm cannibalized surgically. Dr. Xavier (Lionel Atwill) is worried about publicity and will experiment to catch the killer in 48 hours. The police interview the doctors. Wells (Preston Foster) is studying cannibalism, has kept a heart alive, but is missing his left hand. Haines (John Wray), who was in Tahiti when a man died on a ship, does brain grafting. Rowitz studies lunacy from moonlight. Duke (Harry Beresford) is in a wheelchair. Xavier's daughter Joanne (Fay Wray) finds Lee on the fire escape, and he questions her as if he were police.
Taylor's story is on the front page. Lee goes to Xavier's and steals photos; but Joanne gets them back, criticizing him. Her father has gone away. Xavier tells the staff that all are suspected and asks their help investigating; all agree. Lee climbs the drain-pipe and listens to the meeting. The butler Otto (George Rosener) tells maid Mamie (Leila Bennett) she has to wear the victim's dress for an experiment. Xavier hopes to reveal abnormal reactions by checking pulse and nerves. Wells, unsuspected because of his missing hand, operates the machine. The last murder is re-enacted. Xavier says that Rowitz is guilty; but he is found dead. Duke is seen walking. Otto finds Lee in a closet fainted from gas. They give him a drink, and Joanne identifies him as a reporter. She asks Lee to wait on the story, and he agrees to stay for breakfast. Joanne finds her father working at night. Xavier and Haines notice the body was altered. Lee and Joanne chat at the beach. Wells gives Mamie a sedative. Police call Xavier and give him until midnight.
Xavier has Wells handcuff him and the other doctors to their chairs. Otto bolts the door. The full moon affects Wells. He puts on a hand of synthetic flesh and rubs flesh on his head. Wells strangles Otto and re-enacts the murder with Joanne in a nightgown on a bed. He is about to strangle her; but the doctors are handcuffed. Lee fights Wells, throwing a lamp at him and pushing him out the window to his death. Lee calls in the story, giving Xavier credit, and saying Joanne may have an announcement. Lee and Joanne kiss.
Strange medical experimentation leads to murders, but in this story experiments are also used to catch the murderer with a reporter offering comic relief and heroism. Desire for physical immortality causes trouble.