In the bar at the Lakeside Yacht Club a bitter, slightly drunk American novelist Mark Kendrick [Alex Nicol] confesses to an unseen listener that he could easily have kept quiet about events of the last six weeks but he wants Carol to hang.
Six weeks earlier, Mark is writing his new book in a house beside Lake Windermere, England. The house across the lake from him hosts parties most nights and there is one in progress now. His phone rings and Carol [Hillary Brooke], the hostess from the house opposite, asks Mark if he could use his boat to transport some of her friends to the house. She meets them at the dock and invites Mark in. He observes her flirting with Vince [Paul Carpenter], one of the guests who is playing the piano. Mark returns to his boat but has no fuel, so borrows some from Carol's husband Beverly [Sidney James]. He invites Mark back to the house for a private drink. While they play billiards, Beverly talks about how he makes his wealth in the city and reveals that although he is in love with Carol, she only loves herself.
They play until morning. Mark meets Beverly's daughter Andrea [Susan Stephen], then Carol and Vince return saying they have been watching the sunrise. The daughter does not like her step-mother Carol and is angry with her father for remarrying so leaves the room upset. As Mark leaves, Beverly invites him for another drink at the club a few days later.
Back at his house, Mark sleeps off the effects of the party but is woken by a phone call from his agent Harry [Peter Illing]. Harry is disappointed with his lack of progress and puts the pressure on him to produce three chapters quickly. As Mark watches Carol swimming, he ponders getting closer to her.
Mark attends Beverly's drinks party but on the way home afterwards they and Andrea see Carol and Vince kissing in a boat. They put the drunken Beverly to bed. As Mark goes to leave, Carol enters alone and offers him a drink. She thinks he does not like her but although he has fallen in love with her he leaves after one kiss in an attempt to pretend he is not interested.
Harry rings again and insists Mark come to London immediately since the quality of his three chapters is so poor. On his way he calls in on Beverly who is being treated by a doctor [Gordon McLeod] who has given him only one year to live because of his excessive lifestyle of drinking and smoking. Beverly tells Mark he will alter his will to ensure Carol does not receive anything when he dies. Mark confronts Carol about her lack of love for Beverly but she is unmoved and says they have both got what they wanted from the marriage.
In London, Harry tries to convince Mark's publishers all is fine but they opt out and Harry also says he is no longer interested in representing Mark.
Mark returns to Windermere with no money and little prospects. He telephones to speak to Beverly about money but he is away with Andrea and only Carol is at the house. She invites Mark out for a drink where she tells Mark he has no hope of borrowing money from Beverly. As they leave they run into Frank [Hugh Dempster], one of Carol and Beverly's friends. Carol takes Mark home and they are soon embracing.
Mark, Carol, and Andrea watch Beverly put his new boat through its paces on the lake when Frank arrives and indirectly reveals to Andrea that Mark and Carol are having an affair. Beverly's boat engine catches fire but he is unharmed. Carol admits she wished Beverly had not survived so in his anger Mark decides to pack up and leave, revealing to Carol that Beverly is cutting her out of his will and accusing her of plotting his death. He leaves but misses the last train to London. Beverly catches up with him at the station and convinces him to come back and go out on his boat on a farewell party.
As they are about to leave on the boat, Carol arrives to join them. They go out drinking and fishing but later fog engulfs the boat reducing visibility to almost nothing. Beverly asks Mark to steer but in the fog he has to swerve suddenly to avoid a collision with a ferry, causing Beverly to fall over and he is knocked unconscious. While Mark goes to get the first aid kit, Carol pushes Beverly overboard and Mark has no hope of saving him in the fog.
When they get back to the house, Mark cleans some traces of blood from the deck then rings the police to report it as an accident. After the police leave, Mark and Carol try to convince Andrea it was an accident but she does not seem to believe them.
Mark is visited by Detective Inspector MacLennan [Alan Wheatley] and they both go to see Carol. MacLennan says a witness on the ferry claims to have seen Beverly fall but not overboard. However the body is found and the coroner returns a verdict of accidental death.
MacLennan visits Mark again and reveals he has found traces of blood on the launch and he hears Carol has affairs with many men including Mark. Carol arrives as MacLennan leaves and they discuss what to do. Carol convinces Mark to go back to London while she sorts things out and then she will join him.
After six weeks in London with no contact from Carol, Mark tries ringing but the line has been disconnected. He goes back to Windermere but the house is empty and partly boarded up. MacLennan appears and says she left three weeks ago. Outside in the MacLennan's car, Andrea tries to convince Mark to admit it was Carol who killed Beverly but Mark maintains it was an accident. They take him to the house where Carol is now living and he goes in to talk to her. She says she is now happy, but when he threatens her Vince appears and Mark learns Vince and Carol are now married. Carol used Mark and had no intention of joining him. Carol says there is nothing he can do about the murder but Mark says he is going to the police as he nothing left to lose even though he will be hanged as an accomplice. She desperately tries to stop him as he walks away but fails.
Back in the Lakeside Yacht Club bar Mark finishes his confession with the listener, who turns out to be Detective Inspector MacLennan, and they leave together.