When they board the L.A. bound Santa Fe Chief in Kansas City, the four Harrison children have never heard of the Last Crystal or the magic surrounding it. Worried about their father, who has been injured in World War II, they dread a summer with their boring old Uncle James. But J.D., Mary Carol, Robert, and Grace soon discover that this is no ordinary train trip. Before they get half way to L.A. they have crossed paths with a Nazi spy, been asked to deliver a message critical to the allied effort, and one of them has been kidnapped. In a daring rescue effort they leap from the train only to discover that the train has vanished and they have been drawn into a quest for the Last Crystal.
To get home again, they must cross two thousand of miles of prehistoric wilderness and find the crystal. Its healing waters might help their father. They have nothing to guide them but their wits, each other, and a map that only the youngest can read.