Having lost her chance to become the successor of a retired Hero, the passionate Mebuki Kusunoki is left bitter with envy. Sometime later, she and former Hero candidates are gathered once more by Taisha for a new duty. Unexpectedly, it is not another Hero selection.

Instead, Mebuki and the others must assist the Heroes as Sentinels by investigating the treacherous outside world in preparation for humanity's desired comeback. However, their powers are reduced versions of the ones used by the Heroes, which Mebuki feels is making a mockery of her efforts. Nevertheless, she still puts in her all, hoping that one day she will be recognized by Taisha as someone worthy of becoming a Hero.

Despite the dangers that lie ahead, the Sentinels unify under Mebuki's command, completing their challenging duties as they discover the horrific state of their world.