Two boys discover a series of caverns underneath their moon colony home in this futuristic story from the author of the Tripods series. Marty and Steve may live on the moon, but that doesn't mean they don't want to get away every once in a while. So when Steve makes the suggestion to skip school and take a lunar car out to explore the moon's surface like real astronauts, Marty thinks it's a great idea.

But the fun quickly ends when the lunar car crashes, stranding Marty and Steve beneath the moon's surface.

There, in a bizarre, cave, Marty and Steve find the unexpected: a world filled with various plants, food, and life...including a man who supposedly went missing more than one hundred years before. The boys think that they've found the most wonderful place in the galaxy...but they soon learn that the joy comes with a price. The strange creature that is keeping them alive also wants to control them, and when Marty and Steve decide that they want to leave, the creature might have something else in mind.