In the fourth installment of this stunning young adult series, the King family plummets into a nightmare of head-spinning action, as the house and the worlds that lay beyond grow increasingly dangerous. David, Xander, Dad, and Keal have discovered a terrible secret. Now, finding Mom is only a small part of their mission. And time is running out. Using the portals to build an empire, Taksidian wants the house for himself, and there's nothing he won't do to get the family out. The consequences of his meddling reach far beyond the family--to the future of the world itself. The Kings know their survival depends on stopping the bloodthirsty assassin. If only they can find his weakness in time. Most startling of all is their ability to change the path of history. But will their tinkering in time reunite the family and save the future . . . or set mankind on an irreversible course of destruction?