The beheading of international criminal mastermind Fu Manchu is witnessed in China by his nemesis Nayland Smith. Back in England, however, it is increasingly apparent that Fu Manchu is still operating. Smith is quick to detect that the execution he witnessed was that of a double, an actor hypnotised into taking Fu Manchu's place. The villain is back in London, working from a secret base underneath the River Thames. He has kidnapped the esteemed Professor Muller, who holds the key to a potentially deadly solution from the seeds of a rare Tibetan flower.