The Cold Heart.

Nathalie, traumatized by the death of a child she had to look after, is hired by Marti to take care of the little Serge, his son. The boy is abnormal, immured in silence. Nathalie brings him out of his shell and becomes attached to her protege. She feels his life is threatened: serious incidents happen. For Christmas, they are alone, and the child mysteriously disappears. She discovers him in the basement, locked up in a chest.