CBS has confirmed that Mark Harmon will appear in all episodes of "NCIS: Origins" Season 3, which premieres on Tuesday nights in September. Harmon, who originally played Leroy Jethro Gibbs on "NCIS" for 18 seasons, has been an executive producer and narrator for the prequel but only made occasional on-screen appearances. The new season will feature a present-day mystery that ties directly to Gibbs’s early 1990s training at Camp Pendleton, unfolding over the entire season.