Henry, Jessie, Violet, and Benny use to live alone in a boxcar. Now they have a home with their grandfather and they're going on vacation to Dragon's Mouth Cavern. The four Aldens like rock hunting in the are, until Benny finds a sinkhole that leads to the underground caves. While exploring the caves they realized that someone doesn't want them there. But why? Can the Boxcar Children solve this new underground mystery?