In 1954 New England, six strangers are invited to a party at a secluded New England mansion known as Hill House. After being met at the door by the butler Wadsworth, the guests are reminded that they have been given a pseudonym to protect their true identity and asks that they only use that name with the other guests. During dinner, Wadsworth admits a seventh attendee, Mr. Boddy, and announces that each of the guests is being blackmailed:
Professor Plum is a former professor of psychiatry and current member of the World Health Organization whose medical license was revoked because he had an affair with one of his female patients.
Mrs. Peacock is the wife of a U.S Senator who has been accused of accepting bribes to deliver her husband's vote and claims she must pay their blackmailer to avoid a political witch hunt.
Miss Scarlet is the owner of an illegal escort service in Washington, D.C..
Colonel Mustard is thought, at first, to have been blackmailed for scandalous photographs with one of Miss Scarlet's employees, but it is later revealed that he was a war profiteer who made his money from selling stolen radio components on the black market. He now works at the Pentagon on a private fusion bomb (which is revealed later in the film).
Mrs. White is an alleged "black widow" who was drawn in to avoid a scandal regarding the mysterious death of her nuclear physicist husband. She was previously married to an illusionist, who also disappeared under mysterious circumstances.
Mr. Green is a homosexual, a secret that would cost him his job with the State Department if it were widely known.
Finally, Wadsworth reveals Mr. Boddy's secret to the guests: he is the one who has been blackmailing them. As the guests begin to shout at Mr. Boddy, Wadsworth explains that he has gathered all the guests together to confront Mr. Boddy and turn him over to the police. Confronted by Wadsworth's revelation, Mr. Boddy reminds the guests that, if turned over to the police, he can reveal their secrets while in police custody. Mr. Boddy then distributes to each guest a wrapped gift box which, when opened, reveal one of six weapons: a wrench, a candlestick, a lead pipe, a knife, a revolver, and a rope with a hangman's knot. Mr. Boddy suggests that they use the weapons provided to kill Wadsworth and destroy the evidence, keeping their secrets safe. Mr. Boddy turns out the lights in the room, creating a moment of chaos in which someone shoots the revolver. When the lights come back on, Mr. Boddy is lying on the ground and is pronounced dead by Professor Plum.
Everyone denies killing Mr. Boddy, and Wadsworth reveals that he arranged the event in revenge for his wife who had committed suicide after being blackmailed by Mr. Boddy for having Socialist friends. While trying to decide how to proceed, Wadsworth and the guests check on Ho, the Cook, who is found dead in the meat locker with the knife. Upon returning to The Study, Mr. Boddy is gone and is later found dead by Mrs. Peacock in the bathroom from the candlestick. Wadsworth and the guests assume there must be another person in the house that killed The Cook and Mr. Boddy, so they split up in pairs and search the house with the weapons locked in the cupboard. Over the course of search, three others visit the house and are subsequently found dead: a stranded motorist, found dead in the Lounge with the wrench; a police officer investigating the motorist's abandoned car, found dead with the lead pipe in the Library; and a singing telegram girl, found dead in the Hall with the revolver. Yvette, the maid, is found dead in the Billiard Room with the rope.
Wadsworth announces to the other guests that he deduced the identity of the murderer and runs through a frantic re-enactment of the entire evening, scene by scene, with the guests in tow. Wadsworth points out that each of the victims had a connection to one of the guests and were actually accomplices that enabled Mr. Boddy to find out the secrets he later used to blackmail the guests.
The cook had earlier been employed by Mrs. Peacock.
The motorist was Colonel Mustard's driver during the war and knew of his involvement with the black market.
Yvette had worked for Miss Scarlet and had an affair with Mrs. White's husband, which made Mrs. White hate her, and led her to kill her husband. Colonel Mustard's scandalous photographs were of him and Yvette "in flagrante delicto" (caught in the act).
The police officer had been on Miss Scarlet's payroll for his silence.
The singing telegram girl was one of Professor Plum's patients. He once had an affair with her.
The accounting is interrupted by an evangelist at the front door warning "the 'Kingdom of Heaven' is at hand", who is encouraged to leave. Wadsworth then flips the electricity to the house.
At this point, the story proceeds to one of three endings: A, B, or C.

Question: Why is it assumed that there is someone else in the mansion?
Answer: Everyone denies killing Mr Boddy.
Alternative answer: They thought someone else was there because they were all together with the murders took place.