When their father is critically wounded in World War II, the four Harrison children take the famous Santa Fe Chief to stay with an uncle in California. Worried about their father and less than enthusiastic about a summer with their boring uncle, the four soon discover that this is no ordinary train trip. In rushing to get off of the train, a passenger whom they have barely met asks them to deliver a message to someone they will meet at one of the train stops–a message critical to the allied effort. Then, by accident, they blunder into a private car where they meet the beautiful and mysterious Celeste. She shows an usual interest in the youngest, Grace. When the train breaks down in the desert, Celeste lures them to her car. Her plans become clear. She wants to send Grace on a quest to find a crystal holding water from the beginning of time, water set aside for the healing of the earth. Celeste says it will heal their father, but they suspect that she wants it for herself. In trying to escape her, they leap from the train only to discover themselves in the prehistoric West. Their only hope of return is to find the crystal. But to do so, they have to cross thousands of miles of wilderness. They have nothing but each other, their wits, and a map than only Grace can read.