Three girls have been chosen by the great Shinju-sama to be heroes and fight against the destructive beings known as Vertexes—enemies that threaten the harmony and safety of the world. Unsure of when they would be called to duty, Sumi Washio, Sonoko Nogi, and Gin Minowa spent their time idly. However, with the sudden appearance of a Vertex, they realize they have no idea how to fight together as magical girls when they are nearly bested. They manage to defeat their enemy by sheer determination, but in the aftermath of the battle, the three decide to fix their teamwork issues and improve their combat capabilities. But as more enemies appear—and requiring even more power to defeat their nemeses—the girls may find themselves irreversibly changed by the use of their magic. What price will they have to pay to ensure victory, and is it one worth paying if humanity will be saved?

Unaired episode 13 of Choujigen Game Neptune: The Animation, included in the 7th BD/DVD volume.

Akira Fudou leads the life of a typical high school student. After a chance encounter with school bullies, he is rescued by his childhood friend Ryou Asuka. Ryou and Akira catch up over the course of an afternoon that ends with them crashing a party at a nightclub. Initially uncomfortable with the environment, Akira is shocked when violence provokes some of the club's partygoers to transform into demonic monsters. He and Ryou are only saved from the encounter thanks to the intervention of a great demon known as Amon. Amon possesses Akira, and the two together form the powerful Devilman⁠—a being that, due to the influence of Akira's strong empathy for his fellow man, is resolved to keep the human race safe from the threats of demonkind.

Having fulfilled their destiny during the events of Yuuki Yuuna wa Yuusha de Aru, Sanshu Middle School's Hero Club—consisting of Yuuna Yuuki, Karin Miyoshi, and siblings Fuu and Itsuki Inubouzaki—is back in full swing, helping out those in need wherever they can. They have also gained a new member, a hero from the past named Sonoko Nogi. But eventually they notice that someone who should be among them, Mimori Tougou, is missing; any trace of her existence is completely gone, save for the girls' memories. With no leads on Tougou's whereabouts, the girls regain the ability to transform and begin the desperate search for their lost friend. But what they find is more shocking than any of them could ever have imagined, and the consequences of their actions begin to change life as they know it.