The film is about the family history of young François Grimbert before, during and after the Second World War. As a child, the frail son of very athletic parents Tania and Maxime Grimbert wishes he had an older brother to compensate for his own feelings of inadequacy. One day, however, an old acquaintance of his family reveals to him the truth about his tragic family history and that he had an older half-brother on his father's side, both of which had been kept secret since before his birth. Originally, his parents had been Jewish refugees and had both already been married, but during the war, parts of their families, including Maxime's first wife and his son Simon, had been captured and murdered in the Auschwitz concentration camp. The film presents the story in an alternation of past and present.