Gentle Dr. Jekyll (Spencer Tracy) experiments with human nature, believing that each person has both good and evil sides that can be brought forth chemically. Jekyll tests a serum on himself, releasing his vicious alter ego, Mr. Hyde, on 19th-century London. Prowling the town, Hyde ventures to a music hall and encounters Ivy (Ingrid Bergman), whom he takes forcibly as his mistress. When the serum's effects wear off, Jekyll vows never to take it again. But Hyde is not gone for good.