David and Penny's strange new home is surrounded by forest. It's the childhood home of their mother, who's recently died. But other creatures live here ... magical creatures, like tiny, hairy Pog.

He's one of the First Folk, protecting the boundary between the worlds. As the children explore, they discover monsters slipping through from the place on the other side of the cellar door. Meanwhile, David is drawn into the woods by something darker, which insists there's a way he can bring his mother back ...