Tokyo Butei High School is a special institution that trains detectives who use force, commonly known as 'Butei.' In front of Akari Mamiya, who has become a battle sister with the S-rank Butei, Aria, appears her junior, Inui Sakura, an elite student known as a 'Kakuhashi-sei,' who studies both at Butei High and the police academy. Despite being a junior high school girl, she is a top student ranked highly in both academics and martial arts. Due to certain circumstances, she proposes a battle sister contract to Akari. Around the same time, Akari's cousin, Hikari Mamiya, arrives as an intern from Nagoya Butei Girls' School. However, it seems she has some ulterior motives, utilizing the Mamiya family's techniques...?! Join the weakest E-rank Butei, Akari, and her cute yet unique classmates in this fourth installment of a medium-scale action and slice-of-life comedy!