The Gumma Garra Stone - Lainie Jones - Bog

Everything about the summer holiday in the sleepy little seaside town is going wrong. The surf is flat, the weather terrible, and Sam's friendship with Steve and Gemma is falling apart. Then they find the strangely-marked stone and a violent storm sweeps away everything familiar. Plunged into a frightening world, where all they know and value is useless, their survival will depend on the very people they once scorned.