It is now 1939, two years after Sam and Howie solved the giant horseshoe mystery. Sam, age 13, and Howie, age 12, are now involved in another hazardous adventure.

They explore an empty, dilapidated house on their grandfather’s farm. Under the creaky stairs, they find three metal tools with the letters "X, Y, Z" forged on the ends. Beneath the house, in a secret underground room and tunnel, they discover a skeleton holding a metal box. This box, plus two others, contains clues that perhaps will help Sam and Howie as they attempt to solve the "X, Y, Z" mystery.

When they follow the clues, Sam and Howie discover the remains of an illegal “moonshine” operation. They also learn about a historic naval map stolen from a museum in England.

Dr. I. Yanky, who serves as a dentist in a prison, is very interested in this map. Two men dressed in black constantly harass Sam and Howie as they try to solve the "X, Y, Z" mystery.

Follow Sam and Howie on this exciting journey. Will they be able to follow the clues and find the solution to the "X, Y, Z" mystery before the two men dressed in black? What about the strange Dr. I. Yanky? Will the priceless naval map be found? You won’t know the answers to these intriguing questions until you listen to every chapter.