Tom is a smart, talented loner with a chip on his shoulder and a big secret: an imaginary twin on another planet. Eddie is Tom's opposite, a friendly, athletic kid who always looks on the good side. Tom worries sometimes: does confiding in Eddie mean he's nuts? The truth is even crazier than that. Eddie and his planet are just as real as Tom and his Earth, but fifty-some years in the past. And the twins are caught up in an alien master plan that might just mean Earth—both Earths—will be destroyed. Switching places and identities, "slipping" between planets and across decades, a desperate escape, and the unraveling of deeper secrets leave Tom and Eddie aware of the danger they're facing and the tools they can use to overcome it.