London, 1800s. The Whitechapel murderer is on the loose, and kills a young woman before returning to his house. The murderer picks up a pen and begins to write his confession. He is Doctor Henry Jekyll.

Jekyll describes his meeting with his close friend, Professor Robertson, some months previously. Jekyll has been working on a cure for all diseases, but Robertson claims he will be dead before his work could be completed. The thought horrifies Jekyll, who begins instead to research the elixir of life, so that he can prolong his studies for eternity and contribute greatly to mankind. Meanwhile, Susan Spencer and her family move into the flat above Jekyll, and she becomes fascinated with him despite his devotion to his work.

By isolating the female hormones in a fly, Jekyll manages to prolong its life by several days, but in order to investigate the effect of female genes on humans, he enlists the services of Burke and Hare, local grave robbers, to procure female subjects for his experiments.

Burke and Hare struggle to meet Jekyll's constant demand and so begin to kill young women in order to keep up the supply. Jekyll finally becomes ready to test his potion, but upon taking it he finds himself transformed into a woman. Susan's brother, Howard, inadvertently enters the room to discover her, and seeing the article about Hyde Park in Howard's paper, she introduces herself as Jekyll's sister, Miss Hyde.

Once back to normal, Jekyll seeks Burke and Hare for more bodies to refine his formula. However, he witnesses an angry mob attacking their house, having hanged Burke and blinding Hare after discovering their exploits. Jekyll, in a moment of self-doubt, confides to Susan that he is unsure whether to pursue his studies due to the great risks involved. However, Susan's innocent reply that the ends justify the means steels Jekyll into murder so that he can continue to pursue his discovery.

Realising he will be suspected as Jekyll after a poorly executed murder of a prostitute, he becomes Hyde to continue his killings. Nonetheless, Robertson begins to grow suspicious and approaches Jekyll's quarters. Once there he meets Hyde, who returns to Robertson's house and seduces him - before stabbing him to death.

Hyde's personality begins to dominate and attempts to kill Susan, but the urge is resisted by Jekyll. He realises that he must kill for a final time in order to be able to reverse the process and rid himself of Hyde.