1939: Simon Le Tescou is a cruel and tyrannical man. For his 70 years, this rich Breton businessman, decided to reunite his whole family. Edward, his eldest son, who lives at the castle with his wife Edith and dares not stand up to his father. Antonin, the youngest, a deputy on the verge of ruin, whose new fiancée, Madeleine, unexpectedly invites herself. Victor, the youngest, an Olympic champion who accepted Simon's invitation for the sole purpose of settling accounts with his father, whom he has not seen for years. Inès, his granddaughter from Spain, whom everyone did not know existed. And a friend of the family, Eloi, a young African doctor whose research is funded by Simon. But on the night of his birthday, while he refused to feast with his guests, Simon Le Tescou is savagely murdered in his double-locked room.