Following the war, the city is rife with "Extends" - individuals whose bodies have been altered to become lethal weapons. Among them is Inui Juuzou, whose head has been turned into a gun, rendering him amnesiac about his previous self. Interestingly, only someone positioned behind him can activate the trigger of his head-gun. Juuzou earns a livelihood by tackling cases involving Extends in the city, with the help of Mary - a bright young woman whom he has adopted as his ward.