The New York Times bestselling series. On the hunt for the 39 Clues, Amy and Dan Cahill uncovered shocking information about their past - from what happened the night their parents died to the shattering discovery about their family branch.

But the biggest secret of all remains behind Grace Cahill's alternate will.

Now, Amy and Dan are about to discover a long-hidden secret of the Cahills - a secret so dangerous that people died to protect it.

Will this be the end of them? Or just the end of everything they think they know?