While in Tibet researching a mysterious flower that purportedly takes its strength from the moon, botanist Wilfred Glendon (Henry Hull) is bitten by a strange creature. Back in London, Glendon becomes absorbed in his research of the plant, and is annoyed by the appearance of fellow scientist Dr. Yogami (Warner Oland), who reveals that the plant is the only known temporary antidote for werewolves. Glendon remains skeptical until the next full moon, when he undergoes a startling transformation.