A touch of the computer keys, a blast of heat, and suddenly the Murry twins, Sandy and Dennys, are gasping in a shimmering desert. If only the brothers had normal parents, not a scientist mother and a father who experiments with travel in space and time. If only the twins had noticed the note on the door of their mother's lab: EXPERIMENT IN PROGRESS. PLEASE KEEP OUT.
But it's too late for regrets. There's a strange—and very small—person approaching, with a miniature mammoth in tow. At last it's Sandy and Dennys's turn for an adventure—an adventure that becomes serious when they discover that "many waters" are coming to flood the desert. The twins must find a way back home soon or they will drown. But how will they get back to their own time? Can they?