Based on Robert Louis Stevenson's novel, a scientist engaged to be married experiments on himself and discovers an evil alter ego.
In 1887 a man in church laughing at evil being eradicated is committed to a hospital by Dr. Henry Jekyll (Spencer Tracy), who wants to experiment on him but is refused. Late for a dinner, Dr. Jekyll suggests separating good from evil. He kisses his fiancée Beatrix Emery (Lana Turner). Her father Charles Emery (Donald Crisp) advises him to give up his research. Dr. Jekyll and Dr. John Lanyon (Ian Hunter) walk and rescue Ivy Peterson (Ingrid Bergman) from a man. They take her home; Dr. Jekyll carries her in and examines her. Lanyon sees them kiss, and Dr. Jekyll admits it was an attack of evil. Dr. Jekyll experiments with rabbits and rats. He leaves a letter for Beatrix and drinks chemicals. He becomes wild. Butler Poole (Peter Godfrey) knocks, and Dr. Jekyll says Mr. Hyde was there. He rips up the letter, and Beatrix tells Dr. Jekyll she had a bad dream about him. Charles says he is taking her to the continent, delaying their marriage.
Dr. Jekyll gets a letter they are staying longer. He becomes Hyde and goes to a daring show. He sees barmaid Ivy and drinks with her. She pulls away, and he provokes a brawl. Hyde pays the owner to fire Ivy, and he gives her money. Beatrix has not heard from Dr. Jekyll but lies to Charles. Marcia (Frances Robinson) asks Ivy to go out and sees marks on her back. Hyde comes in, and frightened Marcia leaves. He tells Ivy he may have to leave for a while. Ivy wants to go out, but he says no. Hyde forces her to sing.
Dr. Jekyll learns that Beatrix is returning. He melts the key in a stove. Terrified Ivy embraces Marcia and gets fifty pounds in the mail. Dr. Jekyll and Beatrix visit an art gallery, and Charles says they can marry. Dr. Jekyll finds Ivy is a patient, and she shows him her back, blaming Hyde. She asks for poison to kill herself unless he can help her. Dr. Jekyll says Hyde won't trouble her again.
In the park Dr. Jekyll changes spontaneously into Hyde, who calls on Ivy and blames her for seeing Dr. Jekyll. Hyde stops her from leaving, pushes her down, and runs away. Hyde can't get in the back door, and Poole closes the front door on him. Hyde steals pen and paper. Beatrix is disappointed Dr. Jekyll did not come. Poole gives Lanyon a note to get chemicals from the lab. Hyde asks Lanyon for the chemicals. Lanyon has a gun and won't let Hyde leave. Hyde takes the chemicals and changes to Dr. Jekyll. Lanyon calls it blasphemy. Dr. Jekyll goes to Beatrix and says he can't marry her. As he leaves, she cries; he comes back as Hyde, and she faints. Hyde beats a servant with his cane and flees. Hyde bursts in a window, knocks down Poole, and goes in the lab. Police show the cane to Lanyon. Police break in and find Dr. Jekyll. Lanyon says he is their man. Dr. Jekyll denies it but turns into Hyde, who fights until he is shot by Lanyon. Then the corpse changes to Dr. Jekyll, and Poole recites the 23rd Psalm.
Censorship muted the depiction of the frustrated sexual
energy in this version of the dualistic classic with the theme
that goodness is the natural human state, but use of chemicals
can cause the aberration of increased evil as the restraints on
brute behavior are removed.