Henry Hull as Wilfred Glendon, a wealthy and world-renowned English botanist, travels to Tibet in search of the extremely rare Mariphasa lumina lupina, a plant rumored to draw its strength from the moon. There, he is attacked and bitten by a feral humanoid creature, yet he seizes a specimen of Mariphasa. Back in London, he constructs a lamp that simulates moonlight in his laboratory and waits for the plant to bloom, driven by a scientific curiosity and a stubborn hope that this discovery could redefine natural science.
At a party in the city, Dr. Yogami, Warner Oland, approaches Glendon and recalls a brief meeting in Tibet where he too hunted Mariphasa. Yogami insists that the plant can serve as a temporary antidote for werewolfery (lycanthrophobia) and proclaims that he knows two men in London who are werewolves, having been bitten by another werewolf. The claim lands with a mix of intrigue and skepticism in Glendon, who keeps his guard up even as the two exchange notes about potency, antidotes, and the strange connection between blossoming plants and human transformation.
A single blossom opens under the glow of Glendon’s moon lamp. When his hand grows fur in that artificial light, he cuts the blossom and applies its sap to the fur, astonished as the transformation reverses itself. Yogami returns, requesting two blossoms because that night marks the first of four nights around the full moon when the werewolves he mentioned will turn if left untreated; Glendon refuses, misunderstanding the scope of danger, and Yogami warns that werewolves kill what they love most, a line that threads dread through what follows.
Meanwhile, Glendon dives deeper into werewolf lore, learning that the creatures must kill someone each night they transform, or they will not revert to human form. Yogami steals two newly opened Mariphasa flowers, escalating the peril. Glendon chooses not to accompany his wife, Lisa Glendon, Valerie Hobson, and her childhood sweetheart, Paul Ames, Lester Matthews, who have returned to town for a visit, to Aunt Ettie’s party. Soon after they depart, Glendon undergoes a nightmarish change and becomes the very thing he fears. He pursues Lisa, but Ettie Coombes, Spring Byington, screams in fright, scattering the werewolf’s attention, and he settles for killing a random woman in the street. The partygoers hear a loud howl, but Ettie is the only one who claims to have seen a werewolf, leading others to dismiss her tale as drunken hallucination.
Paul, reading about the victim in the newspaper, begins to believe Ettie’s account. He informs his uncle, Colonel Forsythe, Lawrence Grant, the chief of Scotland Yard, but Forsythe dismisses the warning with skepticism. The last Mariphasa blossom has not yet opened, so Glendon rents a room for the night and locks himself inside, hoping to control the urge. He breaks out after transforming and makes his way to the zoo, where he frees a wolf and kills a woman who had come for a nocturnal tryst with the nightwatchman, a grim display of the monster he has become.
With the world unaware of the true danger, Yogami—now without blossoms—pleads with Forsythe to seize the plant from Glendon. Forsythe, focused on tracking an escaped wolf rather than a supernatural killer, takes little decisive action. Glendon travels to Falden Abbey, the estate of his wife’s family, and the caretaker locks him in a room to protect others. Lisa and Paul briefly appear on the grounds, exploring the country house before Paul returns to California, and Glendon, in his werewolf form, glimpses them through a window. He breaks free and attacks, but Paul drives him off, recognizing him despite the danger. Paul relays what he has learned to Forsythe, who begins to entertain the possibility that there are two werewolves at large rather than a single culprit.
The hunt continues as the police search for the hidden wolf-werewolf, and Glendon hides away in his laboratory, waiting for the last Mariphasa blossom to bloom. When the final flower finally opens, Yogami enters and uses it, triggering a furious confrontation with Glendon. The struggle culminates in Glendon’s brutal transformation, his pursuit of Lisa, and Ettie’s fainting fear as the house becomes a maze of danger. Forsythe arrives in time to end the chaos with a gunshot, shooting Glendon as he closes in on Lisa. In a poignant, tragic twist, Glendon dies and reverts to human form. Forsythe offers a grim explanation for the verdict: he will report that he accidentally shot Glendon while the botanist was attempting to protect Lisa.
In the end, the film lingers on the cost of ambition and secrecy—the marriage between science, superstition, and fear—and the irreversible consequences of a man who sought to cure a fear that might have been better left unexplored. The story leaves behind a quiet, unsettling echo: sometimes knowledge demands a price that no laboratory lamp can measure, and even the most well-intentioned pursuit can unleash a darkness that no antidote can fully contain.
