Lily, 11, disappears during her mother's birthday party. At the same party, a little boy, Diego, says he saw Lily's dress burning in a fire without being able to say where. The new gendarmerie captain Romain Sauvant is in charge of the investigation while Sarah, his wife, a school psychologist, must also meet Diego at the request of his school teacher.

Romain seems to have a serious lead in the person of Manu Trabuc, a former associate of Lily's parents. Sarah, alerted by certain visions of Diego, is convinced that the child can help advance the investigation. But when Diego reveals elements of his childhood to her, Sarah understands that he can also help her find traces of her own past of which she has no memory until she was placed in foster care at the age of 4 years old.