Mathilde, 8 ans, vit dans les corons d'un village du nord de la France. Dans la ruelle, où habitent aussi ses grands parents, la vie suit son cours au rythme de l’école et des heures de pointage. Mais mémère semble être de plus en plus bizarre avec son histoire de trésor cachée such’terril.

Mathilde, 8 years old, lives in a terraced house of a coal mining town in the North of France. On her street, where her grandparents also live, life goes on: school on one side, workers checking in on the other. But Granny seems stranger and stranger with her story about a hidden treasure on the slagheap. Mathilde will soon understand that what lays hidden and forgotten is more than just a story about a box.