A young couple, Martin and Tereza, and their son Jakub live a happy life. Though Jakub has Asperger's syndrome, they are unaffected and manage to lead a perfectly normal family life. However, when registering their son for school, Martin realises that something is not right: Jakub has a different bloody type to both his parents. There seems to be only one explanation for Martin, that Jakub is not his son. Tereza must have cheated on him and has been lying to him all these years. But the solution to this mystery is actually far more complicated. Their real son lives in a different town, in the large family of a train driver Karel and his wife Olga. The boy's name is Tomas and he is very much loved by all his family. They too are not aware of the swap. But after a checkup in hospital, it turns out that Tomas is not their biological son.
The boy's swap affects not only the two families with ties to a small town, but everyone else living in that town as well. As the search to understand what happened with the boys goes on, both sets of parents find themselves in a moralistic mire, as they decide what makes a family a family. These heavy questions take a toll on their relationships, while more difficult questions are raised and historical events exposed around them, with more and more people becoming involved to work out what really happened six years ago.