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.
- In 1933, in a castle located high in the Himalayas, master criminal, Dr. Fu Manchu (Peter Sellers), celebrates his birthday by demanding a vial of "elixir vitae," a fountain-of-youth potion responsible for his 168 years of life. When a follower (Burt Kwouk) presents the elixir, his clothing catches fire from the candles on the birthday cake, so he douses the flames with the elixir. Fu Manchu orders the follower tortured and killed, then announces to his other bandit followers, the Dacoits, that they must carry out several international robberies to gather the precious ingredients for more elixir.
In America, a Dacoit is sent to a Washington, D.C., museum, where he observes the Star of Leningrad diamond, and learns that Czar Nicolai presented an identical canary diamond to his cousin, King George V, that is on display in London. At night, the Dacoit steals the jewel.
In London, FBI agent Giuseppe "Joe" Capone (Sid Ceasar) and his assistant, Pete Williams (Steve Franken), visit Commissioner Roger Avery (David Tomlinson) and his nephew and assistant, detective inspector Robert Townsend (Simon Williams), at Scotland Yard to discuss the case. When Commissioner Avery is shown a clue, he immediately recognizes it as belonging to the Si-Fan, the dangerous, secret Asian crime organization headed by Fu Manchu. The FBI agents ask to speak to Inspector Nayland Smith (Peter Sellers in a dual role), an authority on the Si-Fan, but learn that he is retired.
When the Scotland Yard police and the FBI special agents journey to Smith's country home to coax him out of retirement, the retired Inspector examines the evidence and corroborates that the robbery is the work of the Si-Fan. Although Joe Capone believes that Fu Manchu died in 1890, Smith claims that the criminal is alive and well. Just before Smith's retirement, Fu Manchu was responsible for three unrelated diamond robberies. Smith explains that all three jewels represented key ingredients of the elixir vitae that Fu Manchu needs to maintain eternal youth.
Later, Smith agrees to work on the case when he deduces that Fu Manchu is assembling a fresh batch of elixir, and will strike next to obtain the George V canary diamond displayed in the Tower of London. There, museum director Sir Nules Thudd assures Commissioner Avery that the jewel is safe behind bars, guarded by a sophisticated alarm system with several guards standing duty. Meanwhile, as Fu Manchu's strength diminishes, he receives electric jolts to his heart that temporarily restore his vitality.
At lunch, Smith shares his theory that Fu Manchu will kidnap the royal couple and demand the jewel as ransom instead of stealing the diamond outright. He proposes that Scotland Yard plant decoys to outsmart Fu Manchu. The agents hold auditions to recruit the royal doubles from within their ranks to foil the predicted kidnapping, and Constable Alice Rage (Helen Mirren) is chosen to assume the role of Queen Mary. When Alice, impersonating the Queen, makes a speech at the Botanical Society, she and surrounding officials fall unconscious after breathing the vapors of a deadly plant strategically left by Dacoits. However, the kidnapping plot fails when the kidnappers are unable to move the "Queen" because Smith has handcuffed her to a railing.
Soon, King George informs Commissioner Avery that the royal couple plans to attend the theater, so the police alter their strategy. With binoculars, Smith sees several Dacoits waiting to strike, but warns the FBI to wait. When the royal couple appears at their box seats, five sets of identically dressed impostors join them as the national anthem plays. As a result of the confusion, the royal couple is saved and the Dacoits are arrested.
Later, Fu Manchu disguises himself as Charles Rotten, an antique dealer that the Queen is scheduled to visit. When Rotten shows Alice some new tapestries, the undercover agent is kidnapped. She is transferred to a secret location in back of a Chinese restaurant, where she informs Fu Manchu and the Dacoits of her real identity. Before she places them under arrest, Fu Manchu serves Alice a glass of rare and addictive wine. Soon, villain and policewoman are singing a music hall duet when a restaurant worker informs Fu Manchu that Nules Thudd is dining at the establishment. Fu Manchu instructs the Dacoits to have Nules tortured by Dr. Arnold Wretch, an obesity specialist.
Meanwhile, Alice agrees to play the saxophone as Fu Manchu offers her a lifetime of luxury if she becomes his consort. For treatment, Wretch orders Nules to walk on stilts in a park, where Fu Manchu tempts him with a Chinese banquet. Alice, pretending to be an aristocrat hosting the party, introduces Nules to Fu Manchu disguised as chef Fu. Fu suggests that Nules can enjoy the banquet in return for Fu being allowed to look at the King George V diamond. After Fu Manchu has stolen the diamond and the Crown Jewels of England, he informs Nayland Smith by phone that he has gathered all the elixir ingredients and mayhem will follow.
In pursuit of Fu Manchu, Smith and the other agents travel to the retired Inspector's home, which transforms into a hot air balloon that transports them to Nepal. Meanwhile, Alice has joined Fu Manchu at his castle, where he discovers his freshly made elixir is defective. When the Dacoits surround and shoot at Smith's cottage after it lands, Smith demands to see Fu Manchu.
At the castle, Smith confesses that the police planted a fake jewel and agrees to surrender the real George V diamond if Fu Manchu will return the Crown Jewels. When the other police officers arrive at the castle, Smith admits that he saved their lives by giving the real diamond to Fu Manchu. Drinking the improved elixir vitae shaves years off of Fu Manchu's life. Alice distributes the missing Crown Jewels to the British officers and the FBI agents receive the Star of Leningrad diamond. For Smith, Fu Manchu has prepared a small bottle of elixir vitae but warns him not drink it until he returns to England. Fu Manchu also suggests that ingesting the elixir will expose Smith to forces he will not be able to resist. When Smith asks for a clue to prepare him for what might happen, the lights dim and Fu Manchu transforms into an Asian Elvis Presley and launches into a rock and roll song "Rock A Fu" with the Dacoits singing backup.