Fu Manchu's 168th birthday celebration is dampened when a hapless flunky spills Fu's age-regressing elixir vitae. Fu sends his lackeys to round up ingredients for a new batch of elixir, starting with the Star of Leningrad diamond, nabbed from a Soviet exhibition in Washington. The FBI sends agents Capone and Williams to England to confer with Nayland Smith, an expert on Fu. Nayland suspects Fu will kidnap the king and queen and demand the George V diamond as ransom. Scotland Yard recruits Alice Rage to stand-in for the queen. Fu nabs the "fake" queen; Rage becomes enamored of Fu and aids him in his quest for the George V diamond.
Grisly strangulations in London alert Nayland Smith of Scotland Yard to the possibility that fiendish Fu Manchu may not after all be dead, even though Smith witnessed his execution. A killer spray made from Tibetan berries seems to be involved and clues keep leading back to the Thames.
In his remote Asian hideaway the evil Fu Manchu plots the death and discredit of his arch rival, Inspector Nayland Smith of Scotland Yard, as the first step in his plan to become leader of the world's most terrible criminals.
Christopher Lee returns as Sax Rohmer's insidious Asian villain Fu Manchu for the second of his five vehicles. This time Fu Manchu and his army of henchmen are kidnaping the daughters of prominent scientists and taking them to his remote island headquarters. Instead of asking for ransom, Fu demands that the fathers help him to build a death ray, which he intends to use to take over the world. But Fu's archenemy, Nayland Smith of Scotland Yard, is determined not to let that happen…