Where to hide when there's nowhere left to run?

Sandrine is asked to empty her late grandmother's house on a small island near the Normandy coast. She soon discovers that its elderly inhabitants haven't left the island since their arrival as children during World War II. Sandrine can tell they are terrified of someone, or something. Yet, they refuse to leave the island.

What happened to the children from the holiday camp which was suddenly shut down in 1949? And who was Sandrine's grandmother really?