Amy and Dan have managed to rescue one of the Cahills kidnapped by the Vespers, but the remaining hostages are still in peril. Their search for the next ransom item leads Amy and Dan to Turkey and then to Italy where a visit to the site of an ancient disaster changes everything. The Vespers are far more dangerous than the siblings ever imagined and, even worse, it looks like they have an undercover mole posing as a Cahill. Amy and Dan have to face the biggest decision of their lives with nowhere to turn... and no one to trust.

What would happen if you discovered that your family was one of the most powerful in human history? What if you were told that the source of the family's power was hidden around the world, in the form of 39 Clues? What if you were given a choice - take a million dollars and walk away...or get the first Clue? If you're Amy and Dan Cahill, you take the Clue - and begin a very dangerous race.

Amy and Dan find themselves in a new global race but this time, everything’s different. Those closest to them might be working for the enemy, and Amy and Dan are forced to ask themselves how far they’re willing to go to save the kidnapped Cahills. How much can you risk when there’s no one to trust?

After pulling some spectacular heists, Amy and Dan have become two of Interpol's most wanted criminals. So when Vesper One orders them to steal the world's largest diamond, they know they're facing life in prison...or worse. But with the Cahill hostages still in peril, Amy and Dan have no choice but to launch a mission that leads them to an ancient city full of dangerous secrets. With a Vesper mole sabotaging the Madrigals from inside, Amy and Dan have to fulfill their enemy's request before it's too late. Vesper One has developed a taste for killing Cahills, and Amy and Dan aren't going to wait to see who's next.

A surprising twist brings Amy and Dan back to New England for their first mission on U.S. soil since the beginning of the Clue hunt. But being close to home is anything but a comfort. The siblings have learned that there's a mole in the Cahill organization.... And the traitor is one of their closest confidantes. However, this betrayal is overshadowed when Amy and Dan make an even more devastating discovery: the truth about the Vespers' real plan. Burdened with this terrifying information, Amy and Dan are forced to ask an impossible question: Is it more important to save the hostages...or stop the Vespers?