When Tennessee and Chumley are ordered by Stanley Livingston to get back the rare coin with which they accidentally paid the paper boy, they trace the coin's path to a bakery and then to a cookie from that bakery that Chumley ate. How shall they recover the coin? Tennessee and Chumley turn to Phineas J. Whoopee, who tells them about x-rays.
Tennessee and Chumley get jobs at Stonecutter's Jewelry Store. Mr. Stonecutter tells them that they must protect the diamonds or they will go to jail. Rocky Maninoff robs the store and hides out on a ship. While being pursued by the ship's crew, Tennessee and Chumley run into Phineas J. Whoopee, who tells them about diamonds.
Tennessee and Chumley become private detectives. Hotel manager Mr. Hothead hires them to deal with the robberies committed by Slippery Hood, who has eluded every hotel detective. In order to get a better stake-out on Slippery Hood's hotel room, Tennessee and Chumley turn to Phineas J. Whoopee, who tells them about telescopes.