In England, in the 1920s, Janet Smith and her fiancé, George Hastings, travel to a small village to celebrate her twenty-first birthday with her guardian, Dr. Lomas, the village's resident physician. On their arrival, housekeeper Mrs. Merchant welcomes them and advises them to ignore stories and superstitions spread by the villagers, including handyman Jacob and maid Maggie. When Janet introduces George to Lomas as her fiancé, Lomas seems unhappy about their engagement and so George reassures him that they will no longer be expecting him to support Janet. Lomas surprises them by telling them that Janet has not been living on his money, but on her own, and that she is the heiress to a sizable estate. Janet also owns the house and land, although her father’s will stated that Lomas could live there until he dies. Lomas tells Janet that he had not informed her of her wealth as he wanted any potential husband to be interested only in her and not in her money. The doctor adds that he is duty-bound to reveal certain other facts to Janet on her birthday, the following day. The next morning, after Janet and George explore the house and discover a secret, dust-filled laboratory, Lomas insists upon talking with Janet alone. At the end of their conversation, a stunned Janet approaches George and tells him that she cannot marry him and that he must leave immediately and forget her. When George demands to know why, Janet asks Lomas to explain and he leads them both to a mausoleum containing the remains of four centuries of Janet’s family and shows them her father’s tomb, engraved with the name Henry Jekyll. George is familiar with Jekyll and his experiments to separate good and evil elements within a single personality. Lomas explains that he and Jekyll were medical students and friends until madness overtook Jekyll. The local villagers believe that Jekyll lives on as a werewolf that appears every time a full moon rises. George, who does not believe that madness is hereditary, will not permit Janet’s family history to stand in the way of their marriage and insists that they leave immediately and marry. Lomas advises them that they should stay the night and hypnotizes Janet to calm her. Maggie, anxious to return to her home as it is already dark and the moon is full, sets off for the village. During her troubled sleep, Janet dreams that she chases after Maggie and kills her. Later, Janet finds blood on her hands and nightgown and, while looking in a mirror, briefly sees a distorted, evil version of herself. At breakfast the next morning, Jacob suddenly appears carrying Maggie’s dead body and asserts that a werewolf killed her. Lomas tells George that Janet is in no condition to leave, so that night Janet is given a sedative and locked in her room with the window sealed shut. Again, Janet has a dream in which she kills a young woman and again finds blood on her hands. The next morning Jacob reports that another woman has been found with her throat ripped out and accuses Janet of being a werewolf. After Lomas dismisses Jacob, Mrs. Merchant quits, forcing him to go to the village to hire another housekeeper. While Lomas is gone, an hysterical Janet begs George to kill her but she soon regains her reason. Lomas returns with news that Janet is to be summoned to an inquest on the latest victim and has been forbidden to leave the area. That night, Lomas hypnotizes Janet, then leads her out of the house. George follows them to the tomb where he observes Lomas instructing Janet to hang herself over her father’s grave when she wakes up, then witnesses Lomas’ transformation into a werewolf. The werewolf knocks George out and runs toward the village where he kills a young woman. Patrolling villagers hear the woman’s cries and chase the werewolf back to the house. Meanwhile, George recovers and breaks Janet's hypnotic trance just as she is about to hang herself. He explains to her that Lomas is the real murderer and may have framed her father for earlier killings in order to gain control of the estate. When the werewolf returns to the tomb, George attacks him and the villagers, led by Jacob, arrive and thrust a stake through the beast. As the werewolf dies, he regains Lomas’ features.