Dr. Mirakle works in a Parisian carnival, presenting in his sideshow a gorilla named Erik with whom he claims he can communicate. When Erik takes the bonnet of a young woman named Camille, her fiance Pierre tries to get it back but fails. Mirakle offers to replace the bonnet, later sending Camille a replacement.
Little does anyone know that Mirakle has secretly been kidnapping young women and performing experiments on them, injecting them with ape blood in an attempt to find a human mate for Erik. Each experiment is a failure, and the bodies are dumped.
Mirakle remains in Paris and plans to kidnap Camille after she refuses his request to come see Erik again. He has Erik do so, but Pierre--a medical student--has examined the bodies of Mirakle's victims and has discovered they've been injected with ape blood and hears Camille's screams. Erik kills her mother and takes her away.
Camille's blood, according to Mirakle, is exactly the right blood he needs to complete his experiment. However, Pierre and the authorities are now aware of his crimes and are approaching his hideout. Erik however turns on Mirakle and strangles him before trying to flee with Camille. After Erik is killed and Camille rescued, Mirakle's body is taken to the morgue.