Magnum is hired by an attractive woman to find the missing will of her late father, who was recently killed in a suspicious boating accident, which she suspects her brother of being behind, in order to steal the will. Working on the case, Thomas finds that former client Susan Johnson - with whom he was once locked in a bank vault - is now a fellow private investigator, and is working on the same case - for the brother, who suspects his sister as responsible for their father's death. The two P.I.s are soon locked in a battle of wills as they each race to solve the investigation first, but the pair are forced to put their rather love-hate relationship to one side and team up to solve the case of sibling rivalry.
- Tom Selleck as Thomas Sullivan Magnum IV
- John Hillerman as Jonathan Quayle Higgins III
- Roger E. Mosley as Theodore "T.C." Calvin
- Larry Manetti as Rick Wright
- Carol Burnett as Susan Johnson