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...

As he continues his memoirs, Higgins recalls the spring of 1976, when old friend from Sandhurst, David Worth, arrives in Honolulu to attend the funeral of a mutual friend. Insisting that his name is Stanley Sigerson, Sherlock Holmes's most frequently used alias, and that Higgins is Watson, Holmes's faithful friend, David firmly believes that their friend was murdered by none other than Moriarty, Sherlock Holmes's arch-nemesis. In the name of friendship, Higgins indulges David in his investigation, even though his suspicion that David is insane is borne out, and is shocked to discover that David's assertion that their friend was murdered was absolutely correct.

When the King Kamehameha Club is robbed late one evening by thieves wearing animal masks to conceal their identities, Lt. Tanaka becomes so frustrated by the inability of Higgins, Rick and T.C. to agree about what transpired that he enlists Thomas's help in getting their stories straight. Since the men tell widely divergent versions of the night's events designed to show themselves in the best possible light as the evening's hero, it takes Thomas quite some time and effort to uncover the inside man at the club who tipped off the robbers.

An international French detective – the real-life model for a series of best-selling novels by Robin Masters – who is staying at the Estate, asks Thomas' help in locating a missing heir he's trying to track down. Everyone knows the distinguished Inspector from the many novels based on him, and is willing to help the famed detective in any way they can – especially the star-struck Carol – but things turn more dangerous when the investigation leads to one of the world's biggest drug dealers...

Magnum is hired by meek insurance salesman Dan Wolf to locate his father, who he has not seen for thirty years. But unawares to Thomas, the client is actually an impostor hit-man who is using Thomas to help locate his victim, wealthy businessman Theo Wolf, whom he plans to kill to make a name for himself. In the hit, Magnum too is shot, bringing him close to death. Despite his wounds, Thomas becomes hell-bent on going after the hit-man to claim revenge for the dead man's widowed wife and son – and for himself...

Higgins is preparing for a prestigious chess tournament, and the arrival of an important visiting dignitary, the President of Costa Del Rosa, who will be attending it. There have been threats made on the President's life, and Magnum is asked to apprehend those responsible. But both Magnum and Higgins have even more to contend with when yet another of Higgins' illegitimate half-brothers, the flamboyant Don Luis Mongueo, the last of the royal line of Costa Del Rosa, makes an appearance on the scene. But with the plots to assassinate the President, Don Luis emerges as the prime suspect...