The six people from the game "Clue" (Colonel Mustard, Mrs White, Mrs Peacock, Mr Green, Miss Scarlett, Professor Plum) are invited to dinner at a big house, and are met by the butler, Wadsworth.
Wadsworth explains to the guests that they are individually being blackmailed by Mr Boddy, who is also present. Mr Boddy then hands out weapons (the 6 weapons from the game: candlestick, rope, lead pipe, wrench, gun, dagger), and invites the guests to kill Wadsworth to avoid being exposed.
Instead, someone kills Mr Boddy. During the evening, various other people (the cook, the maid, and several vistors) are mysteriously killed one at a time.
It turns out that all the other victims were Mr Boddy's accomplices, and Wadsworth had gathered them all together to give evidence against Mr Boddy's blackmailing. Wadsworth explains how each murder was done. Who did each murder is another question - the movie has three different endings, each with different killers.
Wadsworth: The game's up, Scarlet. There are no more bullets left in that gun.
Miss Scarlet: Oh come on, you don't think I'm going to fall for that old trick.
Wadsworth: It's not a trick. There was one shot at Mr. Boddy in the study, two at the chandelier, two at the lounge door, and one at the singing telegram.
Miss Scarlet: That's not six.
Wadsworth: One plus two plus two plus one.
Miss Scarlet: Uh-uh. There was only one shot that got the chandelier, so that's one plus two plus ONE plus one.
Wadsworth: Even if you were right, that would be one plus one plus two plus one, not one plus two plus one plus one.
Miss Scarlet: Okay, fine. One plus two plus one...SHUT UP! Point is, there's one bullet left in this gun, and guess who's going to get it!