A scientist's investigations into the nature of good and evil turn him into a murderous monster.

In London, in 1887, prominent physician Henry Jekyll incurs the ire of his older colleagues because of his experiments and views on the possibility of separating the good and evil aspects of man's nature. Harry is deeply in love with Beatrix Emery, the daughter of Sir Charles Emery, who likes Harry, but is concerned over his radical ideas and open display of affection for Bea. Harry throws himself into his work and has enough success experimenting with rabbits and guinea pigs to make him confident that the serum he has developed will work for humans. Hoping to try the serum out on Sam Higgins, a man who went mad after being in a gas works explosion, Harry rushes to the hospital but discovers that Higgins has just died. Harry then decides to take the serum himself and is briefly transformed, in both thought and countenance, into an evil alter ego. After taking an antidote to turn himself back to normal, Harry tells his butler, Poole, that the strange voice he heard was a "Mr. Hyde." Just then, Sir Charles, accompanied by Bea, comes for a visit and announces that he will be taking Bea to the Continent to give Harry time to consider his position. Despite his loneliness during Bea's absence, Harry refrains from further experimentation until he gets a letter from Bea explaining that their trip is being extended because of Sir Charles' health. After taking another dose of the serum, Harry again turns into Hyde and goes to a music hall, where he sees barmaid Ivy Peterson, an attractive, sensual young woman whom he had rescued from an attacker some weeks before. When summoned to Hyde's table, Ivy does not recognize him as Harry, but becomes frightened and screams, causing a brawl to erupt among the customers. Hyde later secretly asks the proprietor to fire Ivy and, despite her reluctance, insists on taking her home in a carriage, where he forces himself on her. Some time later, Bea is concerned that Harry has not written to her in weeks, but hides her worries from her father, who decides that she and Harry may marry, after all. Meanwhile, Ivy, who has been set up in a flat by Hyde, lives in constant fear of him. Her friend Marcia is shocked when she accidentally sees welts on Ivy's back, and when Hyde suddenly comes to the flat, he behaves particularly cruelly toward Ivy. Soon Harry learns that Bea has just returned and determines never to take the serum again. He then sends an anonymous gift of fifty pounds to Ivy and melts down the key to the street entrance of his laboratory, which "Hyde" has been using. That afternoon, Harry meets Bea at a museum and is overjoyed that Sir Charles now agrees to their imminent marriage. When Harry returns home, Ivy is waiting in his patient's room because Marcia and her boyfriend had recommended him. Ivy recognizes Harry as the man who was once kind to her, but momentarily has an uneasy feeling about him. When she shows him her scars and he realizes what Hyde has done to her, Harry is ashamed and soothingly promises her that she will never see Hyde again. That night, as Harry happily strolls across the park toward Bea's house, he suddenly turns into Hyde, without having taken the serum. He then goes to Ivy's flat and finds her celebrating her freedom from him. When he repeats words that she had spoken to Harry, she becomes hysterical with fright and screams, but he strangles her to death before the neighbors can summon the police. He then rushes to the outer door of the laboratory but realizes that the key was destroyed. Poole will not admit him through the front door, so, in desperation Hyde goes to Dr. John Lanyon, Harry's good friend. After demanding the medications that work as an antidote, Hyde transforms back into Harry, to John's shock and horror. Harry reveals everything to John, then goes to Bea to break their engagement. She refuses to accept that they cannot be married, and he leaves, then returns as Hyde. She faints, but her initial scream has roused Sir Charles, whom Hyde then beats to death with his walking stick. Now desperate, Hyde pushes past Poole at Harry's front door and goes to the laboratory to take more antidote. As Sir Charles's body is examined by the police, John sees Harry's cane and realizes what must have happened. He then takes the police to Harry's house where they break down the door of the laboratory just after Hyde has taken the antidote and turned back into Harry. Harry says that Hyde was there but left, but in his anxiety under John's accusations that he, indeed, is Sir Charles' murderer, Harry quickly transforms back into Hyde. While attempting to fight off the police and flee, he is mortally wounded, and as he dies, his demeanor changes back into Harry.