In 19th century London, society has developed a split personality: the public façade and the private truth. At St. Jude's Hospital, Dr. Henry Jekyll is striving to refine and perfect a formula to remove the mental affliction from the criminally insane. He goes before the Board of Governors to ask for permission for a human trial. He explains his theory that every person is comprised of both good and evil, and that he has created the ability to separate the two through medicine. Skeptical of the efficacy and ethics of experimenting on patients, the Board angrily denies his plea.
Later that evening, Henry is tardy for his own engagement party to Emma Carew. As speculations and insults arise, Emma proclaims her support of Henry and his experiments, and her love for him. Dr. Jekyll finally arrives and apologizes for his absence; he expresses a fear that his devotion to his experiments will make Emma unhappy. She reassures him that their love will see them through, and then convinces her father of the same.
Later, on the seedier streets of London, Dr. Jekyll and his lawyer John Utterson enter The Red Rat for Henry's bachelor party. The entertainment of the night is actress and prostitute Lucy Harris. After her erotic cabaret performance, she makes her way to Henry's table. Jekyll and Lucy are drawn to each other in a way that promises each of them a great friendship, but before long it is time for him to leave. Before he goes, he gives Lucy his visiting card and asks her to see him should she ever need anything. After he leaves, Lucy is punished for failing to earn any money from him.
Back at Jekyll's residence, it is time for the experiment to begin. Henry has decided to become his own patient, and he eagerly drinks the potion. For a moment, nothing happens...but suddenly, his body is wracked with pain. As he writhes in agony, he undergoes a gradual transformation into the maniacal Edward Hyde, who proclaims his freedom and escapes onto the streets of London. Hyde's evil instincts lead him to Lucy, now seeking a client on the streets. Though gentle at first, he becomes violent and she flees.
Several weeks pass, and Henry has not been seen in weeks- not even by Emma. Lucy comes to call, seeking the help Jekyll had offered; Hyde's advances have covered her in bruises and wounds. Jekyll treats her wounds; she thanks him with a kiss and leaves wishing someone like Henry could love her and save her.
Over the next few weeks, while Jekyll tries and fails to suppress his darker side, Hyde savagely murders the members of the St. Jude's Board of Governors. He proclaims them morally corrupt hypocrites and liars. London is abuzz with fear and speculation as to whom the killer could be.
Concerned for Henry, Emma enters his laboratory and is reading his journal when he angrily catches her. She implores him to remember the love they share, and he begs her not to abandon him. She promises to wait for him, and he returns to his experiments. He knows that he is losing control whenever Hyde surfaces.
Jekyll finds his way to The Red Rat, where he finds Lucy alone; as she notices his presence, he transforms into Edward Hyde. Hyde tells her that he may go away for a time, and threatens her should she not be waiting when he gets back. Despite herself, Lucy is drawn to his voice and dangerous caresses.
Back at the laboratory late at night, Hyde is slumped in an armchair. Utterson enters the unlocked door, looking for Henry. Hyde admits that Jekyll will never return; Utterson is dismayed to see the lab in complete disarray. He demands to see his friend. Hyde chuckles and then agrees. He drinks the potion and transforms into an exhausted Jekyll, right in front of Utterson's very eyes. Jekyll begs his friend to help him, as there is not much time. He asks Utterson to take money to Lucy and beg her to leave town. Utterson leaves, and Jekyll vows to find a potion to rid himself of Hyde.
Utterson delivers Jekyll's letter and money to Lucy at The Red Rat. She lies on the bed and closes her eyes, dreaming of her new life, when Hyde emerges from the shadows and lights a dim lamp. He reads the letter that Jekyll left her. Lucy – both drawn to, and terrified of, him – promises that she wouldn't leave him. He stabs her repeatedly while singing about the sweetness to be found in her face. A shudder passes through him... and it is Henry Jekyll left to stand before Lucy's lifeless and bloodstained body. Back at the lab, Jekyll enters and looks in the mirror. He confronts Hyde, both sides battling for control.
Later, guests appear at the church for the wedding of Henry Jekyll and Emma Carew. As the ceremony begins, Jekyll suddenly doubles over in pain as Hyde emerges and takes over. Hyde grabs Emma and drags her toward the altar. Emma calls out to Henry, who vows that he will never hurt her because he is inside the evil Hyde. Hyde holds her close. After a long moment, he releases her. Hyde begs Utterson to kill him. Utterson holds a blade to Hyde/Jekyll's heart, but cannot harm his friend. Suddenly Hyde/Jekyll falls forward onto the sword. Emma cradles the tortured man. While she weeps softly, Jekyll dies in her arms.