From the introduction:
Once upon a time, a woodcutter so poor had to take his children into the woods and leave them there to fend for themselves. But the children guessed his scheme and laid a trail of pebbles. They knew the forest quite well and drew a map, noting the various landmarks during the walk. Soon, as they started to go deeper into the woods, they felt a sudden weakness.
Now they have to survive into the forest, to return to the hut, to understand why that illness is growing stronger, and finally to confront their father. And maybe during the return, to understand who they truly are.