When Bayport is hit by a rash of vicious pranks, Frank and Joe investigate. They aren't sure the tricks are meant to be funny, especially the mysterious phone tip that leads them to a spooky mansion that happens to be on fire.

After dousing the blaze, the brother detectives find a vital clue. The trail leads them to a bizarre club dedicated to danger. But the pranks may be only a cover for an ultraserious game plan. Soon the young sleuths find themselves playing straight men for a deadly practical joke -- where the punch line is murder.