In 1954, six strangers are invited to a secluded New England mansion. Greeted by the butler Wadsworth and the maid Yvette, each guest receives a pseudonym to maintain confidentiality: "Colonel Mustard", "Mrs. White", "Mrs. Peacock", "Mr. Green", "Professor Plum", and "Miss Scarlet". During dinner, they discover that they all hold government influence before being joined by Mr. Boddy, who has been blackmailing everyone for some time. Wadsworth has called the police to arrest Boddy, but Boddy threatens to expose everyone's secrets if they turn him in. He then presents the six guests with weapons — a candlestick, rope, lead pipe, wrench, revolver, and dagger — and suggests someone kill Wadsworth to protect their secrets before turning out the lights. After a gunshot rings out, Boddy is found on the floor, seemingly dead.
As the guests investigate Boddy's death, Wadsworth explains how he became indentured to Boddy and summoned the guests, hoping to force a confession from Boddy and turn him over to the police. As the group suspect the cook, only to find she was fatally stabbed with the dagger, someone discovers Boddy is alive before killing him with the candlestick. Wadsworth locks the weapons in a cupboard, but before he can throw away the key, a stranded motorist arrives and Wadsworth locks him in the lounge before throwing a key out the front door.
