Ron Pennington, a friend of T.C. and Magnum's on their basketball team, is shot dead when interrupting an armed grocery store holdup, and they discover his son is linked to the murder.

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'.