Cory delves into the Empty World all on his own with just a computer generated guardian for an ally. He knows that if he's ever going to walk again, he needs to unlock the forgotten secrets of medical technology advanced enough to heal him. Meanwhile, the search party looking for Cory has troubles of its own. Lost in an underground city, Christy and the others have to keep one step ahead of the Ancients who are hunting them as they try to track down Cory. But is the guardian who controls those Ancients the same one helping Cory? is the fourth book in the , a science fiction adventure series for kids aged 8-12. If your kids have blown through the , devoured the , or can't wait for the next , make the complete their next read. Grab and rediscover wondrous alien technology!