Billy's mum says he must never go out through the garden gate and explore the dark forest beyond. So, one day, he does exactly that! There Billy meets the amazing Minpins, tiny people who live inside the hollow trees.

But the Minpins are in danger. The terrible, galloping Gruncher stalks the forest, and the Minpins are disappearing in their thousands. Can Billy find a way to destroy the hungry beast, once and for all--or will it gobble him up too?