Based on characters created by Sax Rohmer, the film stars Sellers in the dual role of Fu Manchu, a megalomaniacal Chinese evil genius,[3] and English gentleman detective Nayland Smith.

The story concerns the 168-year-old Fu Manchu, who must duplicate the ingredients to the elixir vitae that extends his life after the original is accidentally destroyed by one of his minions.

When the Star of Leningrad diamond is stolen by a clockwork spider from a Soviet exhibition in Washington D.C., the FBI sends a pair of special agents (agents Pete Williams and Joe Capone) to London, in order to seek the assistance of Scotland Yard as a card from Fu Manchu's organisation, the Si-Fan, has been left at the crime.

Nayland Smith correctly surmises that Fu Manchu will steal the missing diamond's identical twin, held among the Crown Jewels of the United Kingdom in the Tower of London.

Thudd has obesity-related health problems, and has been ordered by the doctor to walk around for 5 mi (8.0 km) a day on stilts.

He is promised access to Fu's outdoor restaurant of Chinese food, and in return, he helps the Si-Fan steal the diamond.

Nayland Smith then uses his flying country house, The Spirit of Wiltshire, to transport himself and his fellow officers all the way to Fu Manchu's mountain base in the Himalayas.

Nayland Smith rejoins his fellow officers in time to see a rejuvenated Fu Manchu sporting an Elvis Presley-type jumpsuit.

In addition to Sellers, the film features Sid Caesar as FBI agent Joe Capone, David Tomlinson as Scotland Yard Commissioner Sir Roger Avery, Simon Williams as his bumbling nephew and Helen Mirren as Police Constable Alice Rage (Mirren sings the Music Hall standard, "Daddy Wouldn't Buy Me a Bow Wow").

John Le Mesurier, who appeared opposite Sellers in the original Pink Panther and The Magic Christian, has a small part in the film as Smith's butler, and Steve Franken, who played the tipsy waiter opposite Sellers in The Party, returns as an FBI agent.

He [Sellers]'d agreed to do a fairly stock Hollywood comedy thriller, similar to The Pink Panther really, playing a detective and a villain.

[12] Roger Ebert of The Chicago Sun-Times gave the film one star out of a possible four, writing that any Sellers movie was bound to have a few laughs "but the story never really involves us [and] the characters aren't all that interesting".

[15] Tom Shales of The Washington Post described the film as "an indefensibly inept comedy",[16] adding that "it is hard to name another good actor who ever made so many bad movies as Sellers, a comedian of great gifts but ferociously faulty judgment.

Manchu will take its rightful place alongside such colossally ill-advised washouts as Where Does It Hurt?, The Bobo and The Prisoner of Zenda".

[16] The film has been criticized for contributing to racist Chinese stereotypes,[17] a charge which has followed the Fu Manchu books from their earliest publication.