On a train to her new middle school, Momoka Sonokawa is preyed upon by a molester, only to be rescued by a girl wielding dual pistols. The girl turns out to be Miou Ootori, Momoka's upperclassman and president of the Aogiri Academy's Survival Game Club, called Sabage-bu for short. Noticing Momoka's aptitude with a pistol, Miou takes a liking to her and tries to persuade her to join the club. In spite of her steadfast refusal, Momoka inexplicably finds herself a member of Sabage-bu.

Sabage-bu is anything but a typical middle-school club. Its members' socially awkward behaviors scare Momoka. Their personalities range from the fanatical to the completely delusional. They do not hold back from walking around public areas wearing ghillie suits or even bungee jumping from the school building. Thrown together with this collection of survival game maniacs, the zealously ordinary Momoka must somehow adjust to her unexpectedly wild school life.