Danny and his friend Joe Pearson discover the entrance to a cave in the woods near their home. Professor Bulfinch has just invented a portable x-ray machine, and he, along with his geologist friend Dr. Tresselt see an opportunity to use the device in the cave. The two adults, along with Danny, Joe, and Irene, enter the cave on an expedition. They make an astonishing discovery, but they encounter a significant problem which prevents them from leaving the cave.
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