The friends Erland and Sven-Erik, both around 50, work in a paper mill in a small industrial town in Sweden. In his spare time, Erland leads a discussion group, “The School of Marriage”, with his wife May at the local church. At a party, Erland develops a sudden and unexpected attraction to and falls in love with Karin, Sven-Erik’s wife. Karin is also attracted to Erland and the two soon begin a wild love affair. Erland tries to find a rational solution to the situation and, in agreement with the two betrayed partners, decides that Sven-Erik and Karin should move in with Erland and May. They set up ten rules for their new life together, rules that are to bring harmony into the situation. But the experiment puts them all to the test and threatens to plunge them into an emotional abyss.