Testing his theory that in every man dwells a good and an evil force, the reserved Dr. Jekyll (Fredric March) develops a formula that separates the two, turning him into a violent ruffian named Mr. Hyde. Thinking he has found the answer to one of life's grandest mysteries, Dr. Jekyll soon realizes he is becoming addicted to his darker self as he unleashes his violent side on earthy dance hall girl Ivy (Miriam Hopkins) and fights Hyde to regain control of his body.