When Trixie Belden and Honey Wheeler find a cut diamond embedded in the dirt floor of the old abandoned gatehouse, they set out to find whoever left it there.

Somebody is stealing diamonds from the jeweler Mohammed Carat! It looks like the culprit is someone on his staff, but who? Is it Luke Smith, the recently hired fitness fanatic with a taste for the finer things in life? It surely can't be Vivian Leander, the loyal shop assistant who is now short of money ... The police in the little town of Valleby are stuck, so they contact young detectives Jerry and Maya, classmates who run their own detective agency....

Amy and Dan Cahill are no strangers to impossible tasks. After seven of their relatives were kidnapped, the siblings scoured Europe for the priceless artifacts Vesper One demanded in exchange for the hostages. But then the Cahill kids are given an assignment that pushes them to their breaking point, a journey to a legendary city. Amy and Dan know they can't afford to fail, yet they're running out of options and running out of time.