The 6:41 train departs from Troyes and arrives in Paris. Like every Monday, the train is overcrowded. Cécile, forty-seven years old, is returning from an exhausting weekend with her parents. She can't wait to see her husband, daughter, and her daily routine again. The seat next to her is free, and Philippe takes it after a brief moment of hesitation. They were lovers for a few months, twenty-seven years ago, but that ended very badly. Without fully realizing it, this episode has significantly affected their lives. As the train speeds toward Paris and silence settles, memories flood back. They have an hour and a half to decide how to proceed...