Higgins volunteers Magnum's services to guard a wealthy socialite's quarantined dog, after the attempted dognapping of the canine. ""Sir Algernon Farnsworth"" soon finds himself to be the most wanted dog in Hawaii, when his former owner, ageing gangster Victor DiGiorgio, has need to get him back. Magnum and an animal regulations officer are soon on the run with Algie from the would-be dognappers - and the fact that the officer seems to be scared of dogs doesn't exactly make matters any easier - but just why does DiGiorgio want the mutt back so badly?...

Magnum returns from a case out of Honolulu, to find that the Masters Estate is being used by a film team to shoot a new movie, based on one of Robin's books. What's more, he is employed to be the stunt double for one of the film's stars, Jack Martin - something which Thomas is highly unenthusiastic about, until he learns that Martin's wife Olivia Ross, of whom Thomas has always greatly admired, is also starring in the film, and relishes the opportunity to get close to her. But things take a dramatic turn when, during filming of a key scene, Olivia shoots Jack with a prop gun, but the bullets turn out to be real instead of blanks, leaving Jack in critical condition. With his idol Olivia the prime suspect in trying to kill her husband, Magnum investigates the ""accidental"" shooting to find just how the bullets came to be changed, and by whom...