In this 7th novel in the Tom Swift Invention Series, the planet Pluto suddenly disappears. It is there one minute and gone the next. Is this the start of an invasion, or something even worse? Tom Swift is called upon to investigate. For starters he agrees to refurbish and move the long-defunct Hubble Space telescope into a position where it might scan the now empty area. Finding nothing he builds an ultra fast space probe. But when that disappears as it nears the missing planet's old position, it raises even more questions. A second probe is flung far to one side by an invisible force. The only thing to do is to go out himself, and so Tom sets about building a new, giant and incredibly fast spaceship, The Sutter. Before he can launch it, Pluto-or something slightly larger-reappears and begins heading inward toward the Sun. The problem is that the Earth is going to be right in the way. If Tom can't deflect this paradox planet from its course, all mankind might perish. But what secrets can this planet hold? And, why does it suddenly slow down inside Jupiter's orbit?