"Yūki Yūna wa Yūsha de Aru: Washio Sumi no Shō" is a prequel to the original "Yūki Yūna wa Yūsha de Aru" series, focusing on the Hero Club's predecessors, known as the Washio Sumi Chapter. The story centers on three middle school girls—Sumi Washio, Sonoko Nogi, and Gin Minowa—who are chosen as "Heroes" by the divine entity Shinju-sama to protect their world from destructive beings called Vertexes. These creatures emerge from the sea and threaten humanity, and the Heroes must combat them using magical powers granted by Shinju-sama.

The narrative unfolds in a world where humanity lives within a protective barrier created by Shinju-sama, shielding them from the Vertexes. The Heroes are tasked with defending this barrier, but their powers come at a cost: each time they transform and fight, they lose a part of their humanity, such as their senses or physical abilities. This sacrifice is known as the "Hero System," and it becomes a central theme as the girls grapple with the consequences of their duties.

Sumi Washio, the protagonist, is a determined and selfless leader who strives to protect her friends and the world. Sonoko Nogi, her close friend, is cheerful and optimistic but hides her insecurities about her declining health due to the Hero System. Gin Minowa, the youngest of the trio, is initially reserved but grows into a courageous and loyal teammate. Together, they form a strong bond as they face increasingly dangerous Vertexes and the emotional toll of their sacrifices.

The series explores their struggles as Heroes, balancing their personal lives with their responsibilities. As the Vertexes grow stronger, the girls must confront the harsh realities of their roles, including the possibility of losing themselves entirely. The story builds toward a climactic battle that tests their resolve and friendship, setting the stage for the events of the original series. Through its focus on character development and the moral dilemmas of heroism, "Washio Sumi no Shō" provides a deeper understanding of the world and themes introduced in "Yūki Yūna wa Yūsha de Aru."