In book 23 of this series, the colonists on Mars have been living in peace and relative comfort, until it is discovered that one of the two small moons racing overhead, Phobos, seems to have dropped in altitude and is continuing to come closer. Since this was not suppose to be at a rate of more than a few inches a year, but is in the hundreds of miles, this is a great worry to them. And, to Tom Swift.He arrives on Mars and uses his repelatron-powered ship, The Challenger, to push the moon back up into position only to find that it starts coming back down. An investigation on the surface of Phobos shows that it has a higher gravity than it should, and the only reason Tom can figure is that the former Space Friends must have planted one of their gravity generating stones somewhere. But, where?And, once he finds it, how can he go about digging more than a hundred feet onto a moon that is barely held together and is composed of rocks and dust gathered over more than a million years. One false dig and the entire thing could break apart.The Space Friends are of no help as they have all but disappeared. So, Tom sets out to do the impossible; replace Phobos in its rightful position once and for all times so the colony can go back to being safe.