The story begins when Joe wants to go swimming; however, Frank points out that there is not enough water because of a low reservoir. When they discover that the water at the Skull Mountain facility disappears each night, they team up with Chet Morton and engineers Dick Ames and Bob Carpenter to solve the mystery. While exploring Skull Mountain, the boys are attacked several times. They finally find Timothy Kimball Jr. (Sweeper) breaking into Kleng’s plumbling store in order to steal the money Kleng owes him ($5000). Kimball is arrested and questioned about the reservoir. This leads the Hardy Boys to catch the villain, Kleng, and solve a crime which involves the Chicago syndicate, being investigated by their father, Fenton Hardy.