When Bayport is hit by a rash of vicious pranks, Frank and Joe investigate. They are not sure the tricks are ment to be funny, especially the mysterious phone tip that leads then to a spooky mansion that happens to be on fire.
After dousing the blaze, the brother detective find a vital clue. The trail leads them to a bizzare club dedicated to danger. But the pranks may be only a cover for a ultraserious gang plan. Soon the young slueths find themselver playing straight men for a deadly practical joke-where the punch line is a murder...