Anna and Pawel Kowalski lead a happy life in Gdansk, Poland, until their 8 year old son Tomek gets hit by a car. Not only is Tomek badly injured, his father proves not to be suitable as a blood donator. Pawel accuses his wife of betrayal of confidence, but she knows that only her husband can be the father of their child. She fears the worst: if Pawel isn't the biological father, she cannot be the biological mother.Worried, Anna searches the hospital archive and finds out that another premature delivery happened on the 15th of Mai in 1989 in that clinic. The other parents are from Dresden, Germany. Anna investigates for their address and drives against her husbands will to Germany. She visits Horst and Maria Schneider who live separated. She tells them what she had found out. For several reasons Anna is not welcome. When Anna tries to make a photograph of Oskar in front of his school, the teacher calls the police. After this incident Anna goes back to Gdansk and decides to leave everything as it is.But now the Schneiders become active. Maria wants to straighten out her relationship to Horst and he sees a chance to save their marriage. Although the Kowalskis don't want a meeting, the Schneiders travel to Poland. At the lakes of Kaschubia the two families and their sons meet. The resemblance with the boys and their real parents cannot be denied. During a two day discussion the parents try to find a solution. They forget to look after their sons, while the boys begin to understand what this is all about. Tomek and Oskar react like typical boys: they run away with a boat. A dramatic night search be water and by air begins.