Botanist Wilfred Glendon (Henry Hull) travels to Tibet in search of a rare flower, the "Marifasa Lupina," which blooms only in moonlight. Despite warnings that the region is dangerous, Glendon continues his quest until finally locating the exotic flower, but not before he has to defend himself from an attack by a howling monster. Back in London, Glendon is visited by the enigmatic Dr. Yogami (Warner Oland), who tells him a current rash of murders is the work of two werewolves. Yogami also claims that the only antidote is the blooming Marifasa flower, which keeps the werewolves from harming the ones they love. Glendon scoffs at Yogami's stories, until the next full moon!