Book Three in the 'Magician's House Quartet' covering an eventful year in the lives of the Constant children, William, Mary and Alice, whose parents, both doctors, are away, working at a hospital in Africa. Consequently the school holidays are being spent with Uncle Jack and Phoebe at Golden House, in The Golden Valley, an area of wild country on the Welsh borders.
Little by little, the children discover that things in this remote old house, far away from anywhere, are not quite what they seem. There are powerful forces at work, calling to the children from across the centuries, and they each have an important role to play in what turns out to be the most exciting adventure of their whole lives.
In 'The Tunnel Behind the Waterfall', the children must fight against evil creatures as they attempt to master time travel.