Five-O was a special state police unit answering only to the Governor of Hawaii. It worked with Honolulu police to fight the underworld in the island state. Sooner or later virtually all the badguys heard Steve McGarrett grunt "Book 'em, Danno!" though chief badguy Wo Fat was caught after MacArthur left the series.
Although based in Downtown Honolulu in the Iolani Palace, the men of Hawaii Five-O are members of the Hawaii State Police, and are accountable to the Governor of Hawaii. Led by Steve McGarrett, its mission is to deal with "organized crime, murder, assassination attempts, foreign agents, felonies of every type".
Hawaii Five-0 is the name of a special police unit consisting of no more than four men at a time, although it can freely call upon the Honolulu Police Department for more men if needed. Answerable only to the governor, Five-0 investigates the most important and sensational crimes, ranging from murder and kidnapping to terrorism and international espionage. Five-0's leader is ex-Navy Intelligence investigator Steve McGarrett, a tough Irish cop possessed with a determined nature, a hatred of crime and an elaborate bouffant hairdo. Other Five-O personnel included Dan Williams, second-in-command and "Dan-O" to McGarrett; pipe-smoking Chinese Chin Ho Kelly, native Hawaiian Kono; Samoan detective Ben Kokua; and Hawaiian Duke Lukela, who worked his way up from patrolman. McGarrett's archenemy was Red Chinese agent Wo Fat, whose plans were always thwarted by McGarrett, and who always managed to escape the reach of the law--at least until the final episode.