A botanist, attacked by a werewolf in Tibet, tries to use the flower antidote he found but becomes a werewolf and murders in London.
In Tibet botanist Dr. Wilfred Glendon (Henry Hull) is warned by a priest about a valley, but he seeks a magical flower. Glendon finds it and is attacked by a werewolf that wounds his arm. In his lab Glendon experiments with the flower. His wife Lisa Glendon (Valerie Hobson) calls him to a party. Dr. Yogami (Warner Oland) asks Glendon if he obtained the flower that lives on moonlight. Lisa's childhood friend Paul Ames (Lester Matthews) asks her what is wrong. Yogami tells Glendon that the flower is an antidote to werewolfery that is caused by a bite from a werewolf. Glendon's lamp makes flowers bloom. His hand becomes hairy; but the flower's juice returns it to normal. Ettie Coombes (Spring Byington) invites Lisa to her party, and Lisa gets Paul to take her. Yogami tells Glendon it is the full moon and asks him to help two souls with his temporary antidote, but Glendon says no. Yogami warns that the werewolf seeks to kill what it loves best. Glendon reads that a werewolf must kill someone at the full moon or become afflicted.
Glendon is transformed into a werewolf and goes out. At Ettie's party Yogami meets Thomas Forsythe (Lawrence Grant). They hear wolf howls. Glendon climbs in the window, and Ettie screams. Glendon attacks a woman on the street. Yogami reads of her murder. Forsythe at Scotland Yard is warned about werewolves. Glendon needs a flower and tells Lisa he won't go on a moonlight ride and tries to stop her. Glendon rents a room, prays, and locks himself in. Glendon is transformed and bursts out the window. He visits a zoo and lets a wolf out. Glendon then attacks a prostitute. Two landladies see Glendon as a werewolf; but they dismiss it, because one was drinking.
Yogami goes to Forsythe at Scotland Yard and tells them to get Glendon's flower to prevent an epidemic. Hawkins (J. M. Kerrigan) tells Glendon that the flower is not blooming. Glendon goes to a monk's rest at Timothy's and tells Timothy to lock him in till dawn. Lisa and Paul walk nearby, and Paul says he loves her. Werewolf Glendon breaks out and chokes Lisa, but Paul clubs him. Paul tells Forsythe it was Glendon. Forsythe investigates a chambermaid's murder where Yogami was seen. Yogami watches Glendon and takes his flower. Glendon says Yogami brought it on him in Tibet, and they fight. Ettie calls Forsythe. Glendon as a werewolf attacks Paul and approaches Lisa, but Glendon is shot by Forsythe. Glendon thanks him and apologizes to Lisa. Glendon dies and becomes human again. Forsythe says he will report that he shot him by accident while Glendon was defending his wife.
This early werewolf film makes plausible the subconscious fear that humans could easily revert to a ferocious animal. Certainly humans can be ferocious, and emotions are more active during full moons. Yet the idea that humans would actually become part wolf is rather far-fetched, though it would become a major source of horror movies.