Diablo-The Devil! The very name of the legendary outlaw strikes fear in the hearts of California ranch owners. Many years ago, the young El Diablo disappeared into a mountain cave, never to be seen again. But the ranchers think he may still be alive — especially now that an eerie moaning sound is coming from his old hideout. The Three Investigators set out in the dark of night to explore the moaning cave — and they soon wish that they were armed with more than a flashlight!