A group of five detectives, each accompanied by a relative or associate, is invited to "dinner and a murder" by the mysterious Lionel Twain (Truman Capote). The guests include:Inspector Sidney Wang (Peter Sellers), a spoof of Earl Derr Biggers' Chinese police detective Charlie Chan, and appropriately accompanied by his adopted Japanese son Willie (Richard Narita). Wang wears elaborate Chinese costumes, and his comically broken English is criticized by Twain and others. Dick and Dora Charleston (David Niven and Maggie Smith) are polished, sophisticated society types modeled on Dashiell Hammett's characters Nick and Nora Charles from the Thin Man film series. The Charles' wire-haired terrier "Asta" is also lampooned, appearing here named "Myron. "Milo Perrier (James Coco) is a spoof on Agatha Christie's Hercule Poirot and he arrives at the house with his heavily-French-accented chauffeur Marcel Cassette (James Cromwell). The demanding, portly Perrier is overly fond of food and appears annoyed that he must share a room with the lowly Marcel, although the two are later seen sharing not only a room but a bed, quibbling like a married couple. Perrier is repeatedly annoyed by being mistaken for a Frenchman as he is Belgian, saying "I am not a 'Frenchie'.
.. I am a 'Belgie. 'Sam Diamond (Peter Falk), a parody of another Dashiell Hammett character, The Maltese Falcon's hardboiled Sam Spade.
He is accompanied by his long-suffering, hard-boiled, sexy but needy secretary Tess Skeffington (Eileen Brennan), whom he continually denigrates and mistreats. Tess Skeffington's name is a riff on Spade's secretary Effie Perine. Jessica Marbles (Elsa Lanchester), a parody of Christie's Miss Marple. Marbles appears as hearty, robust and tweed-clad, wheeling a frail, ancient-looking, seemingly senile companionher ancient "nurse" Miss Withers (Estelle Winwood), for whom she is now caringbut who everyone initially assumes is Miss Marbles. Having lured his guests to his mansion (the address of which is shown early on as "22 Lola Lane" and spoken later as "Two-Two Twain") managed by a blind butler Jamessir Bensonmum (Alec Guinness), and a deaf-mute cook, named Yvetta (Nancy Walker), Twain announces that it is in fact he who is the greatest detective in the world. In order to prove his claim, he challenges the guests to solve a murder which will take place in the house at midnight that very night; a reward of $1 million will be presented to the winner.
Before midnight, the butler is found dead, and at midnight Twain himself appears, also dead; the cook is discovered to have been an animated mannequin, now packed in a storage crate. The party spends the rest of the evening investigating, dining and bickering. They are manipulated by a mysterious behind-the-scenes force, confused by red herrings, baffled by the "mechanical marvel" that is Twain's house and ultimately find their own lives threatened. The ending piles on twist after twist as each sleuth presents his or her theory on the case. After a brutal night during which one pair is almost killed by a snake, another by a scorpion, another by a falling ceiling, a fourth by poison gas and the fifth by a bomb, they all collect in the office where the butler...
believed to have been murdered earlier... is sitting behind the desk very much alive and not at all blind: "The butler did it".
However, each detective then claims that the butler is in fact various incarnations of Twain's associates (such as Irving Goldman or Marvin Metzger) or even his daughter. At first the butler plays the part of each of the persons with whom he's identified, but then pulls off a mask to reveal Lionel Twain himself, very much alive. Twain then disparages each of the detectives (and effectively the authors who created them) for the way in which the plots in their adventures have been handled, including: introducing crucial characters at the last minute for the traditional "twist in the tale" (something the assembled detectives had been doing a few minutes earlier) and withholding clues and information that made it impossible for the reader to find out who had done it. None of the detectives walk away with the million dollars. It is not clear whether any murder has actually taken place. In the last spoken line of the movie, Sydney Wang, when asked by Willie if there had been a murder or not, replies "Yes; killed good weekend!"Alone in his home, Twain pulls off yet another mask to reveal the mute Yetta, who smokes a cigarette and laughs diabolically while rings of cigarette smoke fill the screen.