It was about Magnum and a woman (Patch Mackenzie) working as an investigative reporter discovering crooked interests attempting to conceal the death of a reclusive businessman/industrialist (likely for their own nefarious purposes) whom this woman was in the process in writing a book about. Meanwhile at the same time, TC was making some handsome sums of money doing deliveries to a location that proved, oddly enough, where the crooked interests were located (one of the taller office towers in Honolulu stood in for it, complete with helipad).
Very watchable story even for today, and it features Elisha Cook, Jr in it (first appearance on the show) and also Jon Lormer for laughs. And even Alan Fudge (RIP) playing one of the heavies in his many appearances in familiar movies or TV shows.
Thanks.