The family of the successful writer Erik Sjernborg spends their summer holidays in the Sophienlund manor. Preparations are currently being made for the birthday party of the two twin sons who will come of age. One of the two, Knud, comes with a friend from Copenhagen, where he is studying. The celebration takes an unexpected turn when the parents make a few confessions to their grown children: Sister Gaby comes from a premarital relationship between her mother and a drawing teacher, while the twins come from a brief love affair between the father and an actress. The children are initially completely confused about these new family relationships, which leads to a serious crisis. It comes to a dismaying climax when Gaby and Knud pack their bags to leave the house ...