Ex Vietnam veteran Thomas Magnum (Tom Selleck) leaves the Navy to take up a position as private eye and security advisor to famous author Robin Masters. He is soon living the high life with use of Masters' beachfront Hawaii mansion and Ferrari. But the idyll is not all it seems. There are Higgins, Apollo and Zeus to contend with, Masters' uptight butler and his highly trained guard dogs as well as the other permanent residents at the mansion.

But soon Magnum is drawn into the islands' underworld while investigating the death of his old war buddy, leading him to confrontations with drug dealers, crime bosses and the Navy hierarchy, all dealt with in his trademark cool, laid back style, and loud shirts. With help from friends TC (Roger E. Mosley) and Rick (Larry Manetti), Magnum starts to unravel the secrets of the seemingly quiet island.