Sandy and Dennys have always been the normal, run-of-the-mill ones in the extraordinary Murry family. They garden, make an occasional A in school, and play baseball. Nothing especially interesting has happened to the twins until they accidentally interrupt their father's experiment.
Then the two boys are thrown across time and space. They find themselves alone in the desert, where, if they believe in unicorns, they can find unicorns, and whether they believe or not, mammoths and manticores will find them.