The successful writer Erich Eckberg lives with his wife Sigrid and their three children, the twins Knut and Michael and their daughter Gabriele, on the stately country estate “Sophienlund”. On the sons' 21st birthday, the day they came of age, Eckberg reveals a great family secret to his children: Knut and Michael are not his wife's children. Her real mother - with him as father - died in childbirth, and Sigrid adopted her as her own sons when she married Erich, so that they could grow up in a “real” family. And another secret is revealed: Gabriele is not a “real” Eckberg either, her biological father is an Italian - she was brought into the marriage with Eckberg by mother Sigrid. As different as the temperaments of the three children are their reactions to the revelation. It will be a turbulent night on “Sophienlund”. While the cheerful Gabriele comes to terms with the facts very quickly, Knut simply cannot come to terms with the "deceit" of his parents. Michael, on the other hand, who has always loved his "mother" Sigrid, now goes so far as to ask his father for her hand - but only after he has drunk a little courage. Finally, Knut, who has left home out of indignation, is brought back home by stepsister Gabriele.