In EN LEKTION I KÄRLEK Bergman turned to a light-hearted theme: a comedy that could have been a tragedy, as an ironic voice announces at the beginning of the film. The story opens as gynaecologist David Erneman (Gunnar Björnstrand) is throwing his attractive mistress Suzanne (Yvonne Lombard) out of his office and out of his life. She calls him a naïve bore, which he considers a fitting description. He has in fact recognized that he made a huge mistake when he left his wife Marianne (Eva Dahlbeck), who at this very moment is about to take the train to Copenhagen to marry her former fiancé, the boorish sculptor Carl-Adam (Åke Grönberg). In a flashback we learn that David, Marianne and Carl-Adam were best friends until Carl-Adam suddenly had the idea to marry Marianne. When she didn’t show up at the wedding, he sent David to get her. It was only then that David and Marianne discovered their love.
Before the comedy can take its happy course, we experience a bet in the train about getting a kiss from a “total stranger”, an encounter between David and his pubescent daughter Nix (Harriet Andersson) who is suffering from the “disgusting infatuation” of a girlfriend and the affairs of her parents, and a wild scene of jealousy in a seedy Copenhagen harbour bar.