It centered on a fictional state police force (named in honor of Hawaii's status as the 50th State) led by former Navy officer Steve McGarrett. McGarrett was assisted regularly by State Police officers — Danny Williams, Chin Ho Kelly and Kono Kalakaua. Later, Honolulu Police Department Officer Duke Lukela joined the team as a regular, as did Ben Kokua who replaced Kono. Occasionally, they were assisted by other officers on an "as-needed" basis. During the course of the show, the team was also assisted regularly by: medical examiner Doc Bergman, forensic specialist Che Fong and a secretary.
McGarrett and his team hounded international secret agents, criminals, and Mafia syndicates plaguing the Hawaiian Islands. Most episodes of Hawaii Five-O ended with the arrest of criminals with McGarrett's catch phrase to Williams, "Book 'em, Danno!", with the offense occasionally added after this phrase, such as "-Murder one!".