At the Portal's End (Empty World Series) (Volume 3)

Christy’s father is stranded in the Empty World, and she won’t let anyone stop her from going to rescue him.

With the ever-faithful Trevor in tow, Christy defies her mother’s wishes and ventures back to the Empty World. Her father and Detective Lockhart have been missing for months, and she’s determined to bring them home safely. But trouble-maker Cory has his own reasons for wanting to go back. When he can’t convince his brother to accompany him, he bullies Christy and Trevor into taking him along.

But the search takes them to a mysterious underground city, and they become lost. Even if they find Mr. Walker and Detective Lockhart, will they be able to find their way back out again? And what of the secretive figure who seems to be following them? The city holds all the answers they need...if they can find them.