Deep in vast undiscovered caverns under the mountains of this Earth live tiny people who call themselves the Keepers. They move undetected amongst ordinary people on the Earth's surface due to their size and the speed at which they travel.

The day school gets out for the year, Jeffrey Williams discovers one of these little people who is assigned to watch over him. She places a crystal on him which gives him the power to enter the world of the Keepers, moving at one thousand miles a hour.

Jeffrey and his sister Elizabeth go to the mountains of Wyoming to spend the summer with their grandparents. During their visit, they discover a cave, which leads them to the hidden world of these little people. The Keepers had been expecting them. They had been chosen since birth to visit the great city of crystals, and to perform a special mission only they could. They are guided to Crystal City by five of the greatest warriors in the Under World. Their trek takes them on several detours, in order to find passageways large enough to accommodate the young giants. Every turn in the path seems to bring them new dangers and adventures.