Mystery in the sand: A friendly writer with a metal detector, the mysterious inhabitants of a tower house, and the discovery of a locket in the sand are ingredients for the Aldens' latest mystery. Mystery behind the wall: A young Canadian visitor helps the Alden family solve a mystery involving a lost coin collection. Bus station mystery: The Alden children, isolated in a storm at a small bus station, are led into a mystery centering on a polluted river.