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While in Tibet searching for a mariphasa flower, a plant that reportedly takes its life from the moon, botanist Wilfred Glendon is attacked and bitten by a half man-half beast, and only escapes alive after he stabs the beast.

Home in England, Wilfred ignores his beautiful wife Lisa and completely immerses himself in the study of the flower with his assistant, Hawkins.  Wilfred becomes jealous of the attentions that Paul Ames, a former suitor of Lisa, showers on his wife when he attends a party at their home.

Also a guest at the party is Dr. Yogami, a botanist, who expresses an interest in the mariphasa plant and claims to have met Wilfred once before in Tibet, although Wilfred does not recall the meeting.  Yogami's own mariphasa flowers have perished, but Wilfred refuses to entertain suggestions that he join him in his experiments as he prefers to have complete secrecy, except for Hawkins.  Wilfred maintains his stand even after Yogami informs him that drops from the flower are the only known temporary antidote for lycanthropy, in which a man takes on the form of a wolf.

One day, while forcing the flower to bloom by artificial light, Wilfred notices his hand rapidly changing and becoming hirsute.  After sending Hawkins out, he puts a drop of dew from the flower onto his hand, and the process is reversed.  When Yogami later warns Wilfred that a full moon is approaching, intimating that during full moons werewolves seek to kill that which they love, Wilfred puts him off and refuses to believe the "fantasy."

That evening, however, Wilfred reads an ancient text that informs him that "unless (the mariphasa) is used, the werewolf must kill at least one human being each night of the full moon or become permanently afflicted."  While Wilfred is reading, Yogami steals two blossoms from Wilfred's plant, with which he is able to retain his human form during the night of the full moon.

Lisa then leaves to attend a party at her aunt Ettie Coombes' home, but Wilfred, having grown irritable and sensitive to light, stays home and soon transforms into werewolf in his study.  When he discovers the blossoms missing, he goes out into the streets, arriving at Ettie's home, where he attempts to kill Ettie.  Unsuccessful in his murder attempt, he kills a woman in the street.  Assigned to the case, chief of police Forsythe refuses to believe in werewolves, although his nephew, Paul Ames, does. 

The next morning, Wilfred tries to placate Lisa's foul mood by agreeing to go on a moonlight ride with her, but when he sees that the plant has only one blossom that may bloom too late, he cancels their plans and implores Lisa not to go out during the full moon.  Lisa refuses, however.  After she leaves, Wilfred takes a room at an inn and locks himself in, afraid he will transform and murder his wife.  Despite his precautions, he becomes a werewolf and leaps out of the window.

Wilfred is drawn to the zoo, where he releases a wolf to distract a guard from his girlfriend, then kills the woman and returns to his room at the inn.  With no plants left, Yogami appeals to Forsythe for help, but Forsythe refuses to believe in werewolves and declines.  Once back to his original form, Wilfred returns to his lab but, discovering the bud still unopened, goes to the estate owned by Lisa's parents, who are now dead, and asks the caretaker, Timothy, to lock him inside.

Unknown to him, Lisa and Paul decide to revisit her old home and reminisce about their childhood, and when Wilfred sees them from his room in the tower, he breaks open the bars and leaps out near Lisa.  Paul knocks him out and rescues Lisa, later telling Forsythe that he believes the werewolf is Wilfred.  That night, without blossoms to stop the process, Yogami transforms and kills a chambermaid in his hotel.

During the investigation, Forsythe and Paul find the blossoms in the wastebasket, and Paul, aware that Wilfred was working with the flower, realizes that Wilfred must be a werewolf.  While all the police are on the lookout for Wilfred and Yogami, Wilfred returns to his lab in time to see the bud blossom, but Yogami sneaks in and takes the flower.  Wilfred suddenly realizes that it was Yogami who attacked him in Tibet and, in a struggle, Wilfred transforms back into a werewolf and kills Yogami.  As he heads for Lisa's room, she and Ettie call the police.  When Paul arrives, Wilfred knocks him out and goes after Lisa, but is shot by a policeman, whom he thanks for the bullet, as it was the only way for him to change.  Before he dies, Wilfred apologizes to Lisa for not making her happier and returns to his human form.  Forsythe assures Lisa that his police report will be favorable to her unfortunate husband.