When an unsettling, rhythmic moaning emanates from a long-abandoned bandit hideout, three young detectives are drawn into a mystery that challenges their courage and logic. Jupiter Jones, Pete Crenshaw, and Bob Andrews operate as a professional investigative team, utilizing their combined intellect to solve local enigmas. Their objective is to uncover the source of the sound and determine if the legendary bandit El Diablo truly remains within the cavernous depths. They face the physical hazards of subterranean exploration and the psychological pressure of local superstitions, all while relying on their wits rather than brute force to navigate the dangerous environment.

This work is widely recognized as a staple of the juvenile detective genre, celebrated for its ability to balance genuine suspense with accessible, clever problem-solving. Readers often highlight the effective use of atmosphere and the distinct, well-defined personalities of the three protagonists as the primary drivers of the narrative. The series is frequently noted for its influence on generations of young readers who appreciate the methodical approach to mystery-solving. The premise suggests a classic adventure that relies on environmental tension rather than overt violence to maintain reader engagement throughout the investigation.