In 1845 Paris, mysterious Dr. Mirakle (Bela Lugosi) demonstrates his theories on evolution using his ape, Erik, at a carnival visited by medical student Pierre Dupin (Leon Waycoff) and his girlfriend, Camille Espanaye (Sidney Fox). Unknown to his audience, Mirakle performs experiments in his secret laboratory that involve mixing human blood with that of Erik's. After three prostitutes are found dead from being injected with a strange substance, Pierre is alarmed when Camille goes missing.