In 1845 in Paris, mad scientist Dr. Mirakle abducts young women and injects them with ape blood to create a mate for Erik, his talking sideshow ape. Pierre Dupin, a young, naive medical student and detective, Pierre's fiancée Camille L'Espanaye, and their friends Paul and Mignette, visit Mirakle's sideshow, where he exhibits Erik. Both Mirakle and his servant Janos are enchanted by Camille, who Mirakle plans as a mate for Erik. Mirakle invites Camille to take a closer look at Erik, who grabs her bonnet. Pierre tries to retrieve the bonnet but Erik tries to strangle him. Mirakle restrains Erik and offers to replace the bonnet but Camille is suspicious and is reluctant to give the doctor her address. When Pierre and Camille leave, Mirakle orders Janos to follow them.

One of Mirakle's victims, a prostitute, is found dead in a river and her body is taken to the police station. Pierre wants to examine the victim's blood but the morgue keeper forbids it. Pierre bribes the morgue keeper to draw some of the victim's blood and deliver it to him the next day. Pierre discovers a foreign substance in the blood of the prostitute and other murder victims. Mirakle visits Camille and asks her to visit Erik again; when she refuses, Mirakle sends Erik to kidnap her. Pierre, who is leaving his flat, hears Camille's screams; he tries to enter the room but it is locked. When Erik has retreated, the police arrive and arrest Pierre. Neither Camille nor her mother are found. A police prefect interviews three witnesses: Italian Alberto Montani, German Franz Odenheimer and a Danish man, all of whom state they heard Camille screaming and someone else talking in a foreign language. Camille's mother is found dead; her body is stuffed into a chimney and her hand is clutching ape fur, from which Pierre deduces Erik may be involved.

The police, along with Pierre, run to Mirakle's hideout, where Erik turns against Mirakle and strangles him. When the police arrive, Erik grabs Camille and the police chase him and shoot Janos, who tries to keep them at bay. The police corner Erik on the roof of a small dockside house. Erik confronts Pierre, who fatally shoots Erik, saving his fiancée from peril.