The Case of the Secret Passage

While housesitting at the Funderburg home, Amy Jo and Becky discover a hidden door leading to a series of underground tunnels. Strange markings on the tunnel wall, visible only with a black light, lead them further and further into the passageways where the girl detectives uncover a fiendish plot. Before they can escape the tunnel to warn Chief Higgins, the Pony Investigators are caught in a trap where there seems no possible way of escape.