In 19th century Paris, a maniac abducts young women and injects them with ape blood in an attempt to prove ape-human kinship but constantly meets failure as the abducted women die.
It seems more likely that this is merely recycling the kind of growing obsession of the hero we’ve seen for years in movies like Murders in the Rue Morgue (1932) and Frankenstein Meets the Wolf Man (1943).
This brought him to America at the dawn of the sound era, where he shot Tod Browning's Dracula (1931) and Robert Florey's Murders in the Rue Morgue (1932) before embarking on The Mummy.