The beheading of Dr Fu Manchu is witnessed in
China by his nemesis, Nayland Smith. Back in
London, however, it is increasingly apparent to Smith—now assistant commissioner in Scotland Yard— that the international criminal mastermind is still operating. Smith determines that a hypnotized actor took Dr Fu Manchu's place. The villain kidnaps the esteemed Professor Muller, whose research holds the key to realizing an ancient Tibetan legend. A deadly poison created from the seeds of a rare
Tibetan flower—the Blackhill poppy—is supposed to carry the secret of eternal life. A pint of this poison is powerful enough to kill every person and animal in London. Nayland Smith correctly deduces that Professor Muller received his supply of Blackhill poppy seeds from illegal drug trade. When Fu Manchu cut off the drug trade, the poppy seeds could only be acquired from Hanuman, a warehouse owner who is secretly in cahoots with Fu Manchu. Nayland Smith meets Hanuman in his warehouse to question him on the whereabouts of Professor Muller. Hanuman pulls a gun on Smith, who is able to knock him out and flee. Smith deliberately ignores Hanuman's secretary, whom he recognizes as Lin Tang, Fu Manchu's daughter and partner-in-crime. Lin Tang recognized Nayland Smith and told Hanuman to kill him. Hanuman and Lin Tang go to a secret base under the
River Thames. There, Lin Tang informs her father that his nemesis is interfering with their plans. Meanwhile, Professor Muller has refused to divulge how to extract the poisonous essence from the seeds. Fu Manchu has his henchmen kidnap the professor's daughter, Maria. After watching the execution-by-drowning of one of Fu Manchu's female servants, Professor Muller reveals that the Grand Lama gave key documents to Professor Gaskel during the
Younghusband expedition and are currently locked in a vault in a guarded room of the Museum of Oriental Studies. Disguised, Lin Tang infiltrates the museum and plants a listening device. Their henchmen enter through the sewers and are killed by Nayland Smith and his allies, who discover that Gaskel has emptied the vault. Lin Tan hears all. She and her father escape, emerging in Gaskel's study where Fu Manchu hypnotizes Professor Gaskel. In Fu Manchu's underground lair, Professor Gaskel and Professor Muller translate the Younghusband papers on stabilising the poisonous essence at room temperature. Fu Manchu learns that the Essex village of Fleetwick is under freezing temperatures: The seeds' poisonous properties will persist if used there. A radio broadcast to the entire country announces his return, and demands obedience. He kills everyone in Fleetwick, including the soldiers Nayland Smith sent there to protect it, by spraying the poison from a plane onto 3000 people. Professor Gaskel, his usefulness now over, has been hypnotized into committing
suicide. Nayland Smith and his associates determine the location of Fu Manchu's hideout. They plan to flood it by breaking in through the hidden entrance in Hanuman's warehouse. They confront Fu Manchu and his minions, and a brawl ensues. After the lights go out, Fu Manchu and his followers escape to a Tibetan
monastery with Professor Muller while the River Thames hideout is flooded. Nayland Smith and his team escape the underground hideout via an exit that leads to a graveyard. Nayland Smith and company go to Tibet and find Fu Manchu at a Tibetan monastery receiving Blackhill poppy seeds from the Grand Lama. Professor Muller informs them that Fu Manchu already has all the knowledge and poppy seeds he needs to bring the world to its knees. Nayland Smith reassures Professor Muller by revealing that he has a detonator hidden underneath the poppy seeds in one of Fu Manchu's boxes. Nayland Smith, Professor Muller, and their allies leave the monastery. A frustrated Fu Manchu ponders why Nayland Smith did not take the poppy seeds with him. A few seconds later, Smith's detonator goes off and the monastery grounds burst into an enormous ball of flame. Nayland Smith is riding horseback with his allies and sees the explosion from afar. The film ends with a medium closeup of Fu Manchu fading in over the explosion, and his voice saying, "The world shall hear from me again... the world shall hear from me again". ==Cast==