Within a walled city that separates our world from the kaiju trying to destroy it, humanity fights for survival in this intense new yuri series. Ever since they were young girls, best friends Yorino and Haruka have been training to become Bastioners - soldiers who battle in a walled area that protects humanity from the kaiju trying to break through. But when Haruka is killed during a routine exercise, Yorino must fight even harder to make sure she never loses someone close to her again.