Trixie Belden and her best friend Honey are exploring an old gatehouse when they unearth a diamond hiding in the dirt. Has a jewel thief come to Sunnyside? No secret can stay buried long when detective Trixie Belden is on the case!

Trixie Belden loves to investigate, and this time, she and her best friend, Honey, have their sights set on the old run-down gatehouse. No one has touched the building in years, making it the perfect headquarters for their new detective agency. But far more than cobwebs and dust wait inside because buried in the dirt, the girls discover...a diamond!

How could anyone have forgotten such a precious jewel? Trixie is convinced a thief walks among them. And when a few workers suddenly join the staff at Honey's manor house, Trixie has her prime suspects.

This amateur still has a lot to learn about sleuthing, though. If Trixie can't stop attracting attention, the jewel thief just might set their sights on her.