Erland is a middle-aged paper mill worker who, along with his devoted wife May, runs a successful marriage counseling seminar at their local church, teaching couples how to resolve conflicts through absolute honesty. However, Erland’s own beliefs are put to the ultimate test when he attends a birthday party for his best friend and colleague, Sven-Erik, and falls intensely in love with Sven-Erik’s wife, Karin. Erland and Karin quickly plunge into a passionate, secret affair, resulting in a dual betrayal of both their spouses. When the truth is finally revealed, Erland refuses to resort to a traditional divorce; instead, he constructs an unconventional, "rational solution" by convincing all four individuals to move into the same house and live by a set of strict communal rules. As they try to navigate this forced, open arrangement under one roof, human emotion, sexual jealousy, and social stigma shatter their logical experiment, driving both marriages toward total collapse.