Thomas Sullivan Magnum IV (Tom Selleck), a former Naval Intelligence officer, is a private investigator. He lives in Hawaii, on the estate of multi-millionaire author Robin Masters (Orson Welles). Masters is never around, so the estate is run by Jonathan Quayle Higgins III (John Hillerman), a stuffy Englishman, and former Army Sergeant Major. Higgins has a dim view of Magnum, a view he projects through ensuring that the guard dobermans, Zeus and Apollo, make life difficult for Magnum.
Meanwhile, Magnum is out solving cases in Masters' Ferrari, cases often involving murder and intrigue. In these endeavors, he is aided by his closest friends, Theodore "T.C." Calvin (Roger E. Mosley), a chopper pilot, and Orville "Rick" Wright (Larry Manetti).