After the death of his mother and the end of his father's high-powered career, Zach and his Dad have come to the north of Scotland to live rent-free in Grandfather Alistair's cliff-top cottage. Dad asks Zach to clean out the old garden pond, a rotting nightmare where not even a tadpole can survive. But when he drains the pond, he unearths something unexpected- a trapdoor leading down into darkness... He ventures down there and discovers something amazing - a glowing egg-shaped stone. Once brought back to the surface, Zach's run of bad luck seems to change entirely. Suddenly, he can't seem to stop winning and even Dad's career is unexpectedly back on track. But good luck can't last forever... The stone belongs to a race of ancient creatures that dwell deep beneath the ground - and they want it back. Pretty soon, unspeakable things begin to claw their way to the surface in search of the stone - and for Zach and his new friend, Pepper, there's one all-important question. Can they stay alive until morning?