The Fiendish Plot of Dr. Fu Manchu (1980)

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.