When Thomas and old-school private eye Luther Gillis encounter each other in a seedy hotel room with a dead drug dealer as they separately investigate the whereabouts of Nancy Perkins, a St. Louis runaway, they decide to team up to find the missing girl after the police burst into the room and arrest them on suspicion of murder. After Thomas finds out that Luther is actually Nancy's father, Higgins poses the question that leads Thomas to solving the drug dealer's murder and reuniting father and daughter.

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...

Robin and Higgins simultaneously ask Thomas to do some investigative work, so Thomas calls on Luther to nail the man who's blackmailing Higgins over information that could have repercussions for him and an old flame in the House of Commons, while Thomas goes undercover to investigate embezzlement in a bank owned by Robin's friend.

At the 14th Annual Convention of Private Investigators, Magnum is to receive the 'Local P.I. of the Year' award, but the event is brought to a halt when Jean Claude Fornier, the famous French Detective, suddenly drops dead while giving a speech, after being poisoned. Magnum is helped - or rather, hindered - by several fellow crime-busting acquaintances as he seeks the murderer, and becomes entangled with a gang of drug smugglers, that the Inspector was hunting before he was killed...

When Higgins asks his help in finding Robin's latest manuscript after it is stolen, Thomas gets some unexpected help in the case when St. Louis P.I. Luther Gillis arrives unannounced on vacation. They set about trying to work out who is so desperate to get hold of the manuscript that they are willing to resort to attempting to kill Higgins and drugging Zeus and Apollo to get it, in scenes reminiscent of those portrayed in the stolen book...