In Bel Air, California, film gossip columnist Sheila Green storms out of a party at two o'clock in the morning to a darkened street, where she is struck down by an unidentified car. On the anniversary of her death, her wealthy widower, the occasionally sadistic film producer Clinton Green, invites six friends to spend time with him on the Riviera aboard his yacht named for his late wife. Invitations are sent to the actress and sex-symbol Alice Wood; her manager-husband Anthony; the brash and promiscuous talent agent Christine; Philip, a middle-aged, "has-been" director now shooting commercials featuring children; Tom, a screenwriter whose career has been reduced to rewriting other people’s scripts; and his wife, Lee, who is the daughter and heiress of a deceased film mogul. All but Lee, who claimed to be out of town that evening, were present at Sheila’s last party. When his guests arrive, Clint makes a show of carefully posing them under the letters of Sheila on the side of the yacht for a group picture. After they have settled in, Clint, while dispensing piercing comments about their lagging careers, mentions his plans to make a film about Sheila’s life. A great lover of puzzles and competitions, Clint explains that the entertainment for the week is a mystery game he has devised, which he calls the "Sheila Green Memorial Gossip Game.” To begin, Clint hands each person a small card, on which is typed what he calls “pretend pieces of gossip," the kind of “dirty secret” his wife would report in her column. Every card is different, Clint says, and should remain hidden from the other players. He explains that each day he will announce one of the secrets to the whole group, along with a clue, and the players will then try to determine who among them possesses the card with that particular secret. Before sending them by launch to the nearby port, Clint says that one of the guests is “a shoplifter” and provides each person with a silver key. On shore, the guests disperse to solve the mystery individually, and some come to realize that the key unlocks a certain hotel room in which Clint waits with more clues indicating that, for the sake of the game, Philip is the shoplifter. Being innocent of that crime in real life, Philip is amused by this pronouncement, but Alice later confides to Tom that she was once arrested for stealing a coat, and wonders if Clint's game is a way to make public each person’s embarrassing secret. Later that night, Clint and Christine have sex in his stateroom, where she discovers that his sound system is wired to allow him to eavesdrop on conversations in other rooms. On the second evening, Clint arranges for the group to visit an abandoned island monastery, where they must determine which of them is a “homosexual.” Several participants discover that Clint is hiding in a confessional box dressed to look like Alice, who was assigned the card. However, the next morning, Clint is not on the yacht and eventually the others return to the monastery to search where he was last seen. They find him dead, apparently killed by a falling piece of a column, but soon realize that he was murdered by someone among them. As the nearest police are several hours away, the guests wait, speculating about what happened. Questioning whether Clint’s game was the motive for murder, Tom convinces the others to share the contents of the unknown cards, which read “ex-convict,” “informer” and “little child molester.” When Tom reveals that the card he was given reads "hit and run murderer," Lee, a closet alcoholic, confesses that one year ago she was inebriated and driving to the party, when she lost control and accidentally ran over Sheila. Lee explains that she believed Clint suspected her, so she killed him. Lee goes to her room, and when Philip, troubled by details in her story, later goes there to talk to her, he discovers that she has apparently committed suicide by bleeding to death in a bathtub full of water. To await the inquests, most of the guests move ashore to hotels. As Alice departs, she breaks off the clandestine affair she is having with Tom, claiming she has lost interest in him now that he is “available,” and urges him to call her if he remarries. That evening, after the crewmen disembark for a night on the town, Tom returns to the yacht, where Philip, deep in thought, has remained onboard. Perplexed by several inconsistent details that he relates to Tom, Philip confides his belief that Clint was already dead when Lee struck him. Then, recalling something Clint said, Philip retrieves the photograph taken the first day and the cards given to the players, and discovers that the first letter of each card corresponds to a letter in Sheila’s name—except there is no card for “A” and Tom’s “hit and run” card makes an extra “H.” Philip also recalls that on the first day Tom crumpled his card after seeing it, but notes that the “hit and run” card is smooth. From this, Philip suggests that Clint had given him a different card beginning with ”A,” which he guesses was “alcoholic,” because of Lee’s “dirty secret.” Noting that none of the cards written by Clint were worthy of silencing him, Philip concludes that he devised the game for his own amusement and not to reveal Sheila’s killer. When Tom admits that he knew about Lee’s car accident, Philip, aware of Tom’s affair with Alice, guesses that he had tired of Lee, but not her fortune. Philip then outlines how Tom switched the “alcoholic” card with the “hit and run” card that he typed himself and left where Lee would see it, wanting her to think that Clint held her responsible for Sheila’s death. Philip speculates that Tom, after killing Clint, propped him up inside the confessional. Unaware that he was already dead, Lee struck Clint’s lifeless body and Tom allowed her to believe she caused the producer’s death. Philip suggests that Tom later drugged Lee’s drink, and when she was unconscious, carried her to the tub and slit her wrists. Claiming that Philip knows too much to live, Tom tries to strangle him, but is interrupted by Christine and a crewman, who were having sex in Clinton’s stateroom and eavesdropped on their conversation. Recovering quickly, Philip claims that they were discussing how the $5 million Tom will inherit from Lee is enough to finance a “big budget” movie to tell Sheila’s life story. Philip and Christine agree that she will cast the film and he will direct, and that Alice should portray Sheila. As for the script, Philip believes they should get someone outside their circle to author it, but that Tom will be available for rewrites. Resigned to being blackmailed, Tom does not argue.