The Alden children are visiting a gem mine that is holding its annual gem-hunting competition. With a little instruction and the right gear, the Aldens have everything they need to begin their search. It’s not long before Jessie makes a fabulous find. But just when she’s about to win the contest for finding the biggest ruby, her gem disappears! Now instead of digging for rocks, the Aldens are digging for clues. Who wants to win the contest so much that they would steal from the Boxcar Children?