After class on the Yale campus, Frank and Bart visit the post office, and talk about their expected outing to a cavern in the country. The trip to Echo Cavern will help with their geology studies. Suddenly, they find their friend George is hurt. Does he need to be taken to the infirmary? Why does George want to keep his ordeal kept quiet?
At the cavern, Frank enjoys observing the stone formations, and is curious about a locked iron door. Their farmer guide tells about Henderson and his practice of selling tickets to visit his cave, and had been letting his patrons into this side of the cave;
Accusations come out that George had stolen a secret formula. What does it do? How important of a breakthrough is it? Not jumping to any conclusions, Frank gathers more clues about the formula, the whereabouts of George, and that iron door. How does it all fit together? Is there a greater mystery afoot? When Frank and Bart get trapped behind that iron door, it takes some creativity to break out. Now its time to catch a crook.