St. Louis-based Private Investigator Luther Gillis returns to Hawaii, accompanied by his secretary Blanche, to attend the annual Private Investigators convention, where Luther has been informed he will receive an award. But while Luther and Magnum attend the convention, Higgins and Blanche are kidnapped from the Estate, and the two P.I.s find themselves having to join forces once again...

An old comrade of Higgins' brings his travelling carnival to Hawaii. But when one of the performers is fatally stabbed, Higgins asks Thomas to investigate. Working undercover as a carnival 'roustabout', aided by Rick, Thomas learns that even before the killing, the carnival had been experiencing a string of 'accidents', and must pin-point exactly who is trying to force the show out of business...