In the bar of the Lake Windermere Yacht Club in England, American hack novelist Mark Kendrick tells an unseen listener about recent events in his life: One night, Mark, who has been renting a bungalow on the lake's shore, receives a phone call from Carol Forrest, a neighbor who lives across the lake, to ask if he would use his launch to ferry four guests to play a party as her launch has broken down. Mark agrees and is invited to the party at the lavish estate, but does not stay long. As he is leaving, he encounters Carol's husband Beverley, a financier, who invites him to a game of billiards which lasts until dawn. While the house staff is cleaning up, Mark meets Beverley's daughter Andrea, who becomes distraught when Carol, her stepmother, returns from watching the sun come up with her latest lover, Vincent Gordon. Later, Beverley invites Mark to the launching of his new boat. There Carol makes it clear to Mark that she would like him to be her new lover, but Mark resists her. On a trip to London, Mark learns that both his publisher and agent have decided that they will no longer support him, leaving him short of cash. On his return to the lake, Beverley tells Mark that a heart condition will probably cause his death within a year and that he intends to eliminate Carol from his will. Later, Mark attempts to see Beverley about a loan, but falls into the clutches of Carol, who seduces him. The following day, when Beverley is speeding in his new launch, it catches fire. He is saved, but Carol tells Mark that she had hoped Beverley would die. After informing Carol that she is to be cut out of her husband's will, Mark attempts to extricate himself from the situation and leave the area, but misses the train. Beverley encounters him at the railway station and invites him on a boat excursion until the next train. Carol invites herself along. A heavy fog rolls in and Mark, who is steering, has to swerve suddenly to avoid a ferry, accidentally throwing Beverley to the deck and knocking him unconscious. Carol suggests to Mark that they push him overboard, but Mark refuses and goes below deck for a first aid kit. Carol then pushes Beverley into the lake and he drowns. Andrea does not believe her father's death was an accident, and although Detective Inspector MacLennan reports that the skipper of the ferry saw Beverley fall to the deck, not overboard, the coroner returns a verdict of accidental death. As she is still in Beverley's will, Carol suggests to Mark that he leave to avoid further suspicion falling upon them and that she will join him in a month. However, when Carol has not arrived after six weeks, Mark returns to the estate to find it deserted. MacLennan tells him that Carol left three weeks earlier and admits that he has no case against them. MacLennan then drives Mark to a nearby house where he discovers Carol with Vincent Gordon and learns that they have been married for three weeks. Realizing that he has been duped, and despite Carol's pleas, Mark meets MacLennan in the bar, tells him the whole story and leaves in his custody.