Jupiter Jones, Pete Crenshaw, and Bob Andrews are spending time at the Dalton's Crooked-Y Ranch when the cave on the mountain begins moaning after fifty years of silence. Wrapped in the legend of El Diablo, an outlaw who hid in the cave and never returned, the cave holds more secrets than just a legend. With two ancient prospectors, a man with a scar and eyepatch, and man in a speeding car from Nevada, The Three Investigators soon discover the danger is far greater than a mere moaning in the cave. Jupiter soon realizes that time is of the essence, but can they solve the case in one night?