Mathilde (Louisa Pili
), nine, lives with her mother Hélène (Sandrine Bonnaire
) since her parents’ divorce and her grandfather Paul’s heart surgery.
Mathilde's poor grades in school are a sore point with her mother, and Mathilde spends much of her time under her bed in a secret, mysterious world of her own. One night, she sneaks out of the house and makes her way to the nursing home where her grandfather lives. In her hands, she has a letter from Madeleine, Paul's wife, who supposedly died 30 years ago. Mathilde has found out it's a lie, that her grandmother isn’t dead. She's here to spring Paul so he can help her find Madeleine.
Meanwhile, in her daughter's hideout, Hélène is shattered to find a stash of letters from her mother that her father had always kept hidden from her. She jumps in her car and sets off to find them.
And so Paul, Mathilde and Hélène journey for truth begins. Barriers come down, the lies and secrets are unveiled, and thanks to Mathilde's fortitude, they begin to talk to each other.