Magnum, P.I. follows the story of Thomas Sullivan Magnum IV (Tom Selleck), a former Naval Intelligence officer turned private investigator. He resides in Hawaii on the estate of wealthy author Robin Masters (Orson Welles), who is rarely present.
The estate is managed by Jonathan Quayle Higgins III (John Hillerman), a strict Englishman and former Army Sergeant Major. Higgins holds a low opinion of Magnum, which he demonstrates by using the guard dobermans, Zeus and Apollo, to create obstacles for Magnum.
Despite Higgins' disapproval, Magnum is busy solving cases in Masters' Ferrari, often involving murder and intrigue. He receives support from his closest friends, Theodore "T.C." Calvin (Roger E. Mosley), a helicopter pilot, and Orville "Rick" Wright (Larry Manetti).