In 19th century Paris, the mysterious Dr. Mirakle uses his ape, Erik, to demonstrate his theories on evolution at a carnival attended by medical student Pierre Dupin and his girlfriend, Camille Espanaye. 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.