This is the 4th book in the Gregor the Overlander Series. Gregor is visiting his mother in the Underland. She cannot leave until she is fully recovered from the illness she contracted in The Curse of the Warmbloods. While there, Gregor is drawn into a quest with Luxa to save the nibblers. They find the Marks of Secret near two scenes of death but aren’t sure what to make of it. As they are drawn deeper into the Underland, they face fierce opponents who will stop at nothing to destroy them. The only problem with this book is that it leaves you with a terrible cliffhanger! So get the 5th book at the same time as you get this one so you can just keep on reading.