A supposed wealthy teenage ranch owner hires Magnum to stop a dangerous group of cattle rustlers on Hawaii's Big Island, but Magnum finds that his employer isn't quite all he seems and that the neighboring ranch owner implicated of being behind the cattle theft is a business partner of Robin Masters'.

Magnum is called back to service by Admiral Hawkes, who has reason to suspect that there is a security leak at his base that is leaking coded information, and Magnum's prime suspect is the fiancee of the Admiral's son.