This story of love, loss, and betrayal follows a girl named Nine and her friends as they try to save their world from destruction, encountering a twisted church, an evil king, and creatures attacking everything.

From darkness, Nine steps into the world, sent to find a threat to the powerful Gates that lead to all other planes of existence. She is accompanied by her longtime partner, Kai. The two live at first as criminals in District One Hundred Seven, a city surrounded by large stone walls, until they're caught and join the military. When Nine kills another soldier, distrust of her grows. Nevertheless, she and Kai take on extra training and end up making several new friends. Nine and Kai are then split up and sent on separate missions where they each face deadly enemies and vicious creatures. Nine has always known that she is not human, but one of a race called Gate Keepers charged with guarding the Gates, and she has the power to summon weapons and to move at unnatural speed, as well as to heal herself and others. After several incidents, she has no choice but to tell the humans her secrets. Through thrilling adventures and hair-raising danger, she and her friends endure war and torture in order to save the world and help humanity.