Tokyo Butei High School is a special institution that trains detectives who use force, commonly known as "Butei." Akari Mamiya, a junior who admires the ultra-elite of the Assault Department, Kanzaki H. Aria, is a dropout whose only assets are her energy and brightness. Determined to become a "Sister Warrior" under the system where seniors and juniors pair up like sisters, Akari challenges Aria for the right to become a Sister Warrior! The difference in their combat abilities is clear, but Akari has a secret technique...?! The newly formed odd couple of the strongest S-rank Butei Aria and the weakest E-rank Butei Akari, along with their cute and unique classmates, create a medium-scale action and slice-of-life comedy!