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.

Thirteen-year-old Grace already has too much going on — grieving over her father’s mysterious death, dealing with her distraught mother’s erratic parenting, and evading her creepy nosy neighbour, Mr. Stuckless, just for starters. She and her friends Fred, Mai Ling, and Jeeter like to get away from it all by hunting for fossils near their secret hideaway, the abandoned mine they’ve nicknamed The Black Hole. But when Grace receives a strange note regarding her father’s death, it sets off a chain of events that sees Grace and her friends turning into detectives to solve the mystery behind his suspicious disappearance. As the clues and suspects start piling up and the investigation becomes more and more dangerous, Grace and her friends find themselves racing against time through treacherous sinkholes and abandoned mine shafts to figure out what really happened to her father.