Fred has had a rough summer. His secret crush on Mai is going strong, his mother has barely recovered from a battle with cancer, and his unreliable father's diving business has gone completely underwater.

Now Fred, Mai, and Grace, extraordinary fossil hunters, are at the Fortress of Louisbourg hunting a different kind of treasure. They are secretly excavating the historic site, trying to find a mass of jewels Fred's ancestor may have buried there—jewels that could save Fred's family. But Fred uncovers far more than he bargained for, including a dangerous plot that could leave Fred's family in even more serious trouble.

The young detectives from the bestselling The Fossil Hunter of Sydney Mines shine in this fast-paced mystery for middle readers.