Siblings Amy and Dan reach a breaking point when they discover the Vespers' fatal plan (Trust). In Doom, the two face the near-impossible task of destroying a Doomsday device and shutting down the Vespers indefinitely. The final two entries in this suspenseful series are more focused and deliberately paced than previous books, which helps bring the story to a satisfying conclusion.

This fifth book in the 39 Clues: Cahills versus Vespers series is the work of Newbery Medal winning Park and follows young teens Amy and Dan Cahill as they continue their worldwide quest to protect and preserve their large and illustrious family. It turns out that there's a mole within their close-working group (thus the title), so off they go to Yale University (and beyond) to follow clues and get to the bottom of things. After one of their suspects pronounces that the brother-sister team doesn't know a "rat's ass" about loyalty, things get mildly violent, with a few gunshots; then tricky, with a couple boyfriend/girlfriend issues; and then dramatic, with some hostage-release attempts. All of these elements help to make this series popular beyond those interested in the more cerebral components, such as Ivy League medievalists and rare manuscripts. The corresponding online game and accompanying game cards (in the non-library edition) don't hurt popularity either, and fans will be carried right along with a few answers and quite a cliffhanger until a different author produces the next volume.