"The Asterisk War: The Academy City on the Water" is set in a futuristic world where humanity has developed advanced technology and integrated it with supernatural abilities known as "Genestella." The story takes place in Rikka, also called Academy City on the Water, a floating metropolis that hosts the prestigious Seidoukan Academy. This institution is one of six schools that participate in the Festa, a grand martial arts tournament where students compete to showcase their combat skills and Genestella abilities. The winner gains immense prestige and the opportunity to fulfill a single wish granted by the governing body, the Integrated Enterprise Foundation.

The protagonist, Ayato Amagiri, transfers to Seidoukan Academy with the goal of finding his missing sister, Haruka, who disappeared under mysterious circumstances. Ayato possesses a powerful Genestella ability, but it is sealed due to its overwhelming nature. Upon arriving at the academy, he encounters Julis-Alexia von Riessfeld, a skilled duelist and princess of a small European nation. Their meeting is marked by a confrontation, but they soon form a partnership to compete in the Festa. Julis seeks to win the tournament to secure resources for her war-torn homeland, while Ayato hopes the competition will provide clues about his sister's whereabouts.

As the story progresses, Ayato and Julis form alliances with other students, including Saya Sasamiya, Ayato's childhood friend and a skilled marksman, and Claudia Enfield, the student council president with her own hidden agenda. The group faces numerous challenges, including rival schools, political intrigue, and the machinations of the Integrated Enterprise Foundation. The tournament serves as a backdrop for exploring themes of ambition, loyalty, and the consequences of wielding immense power.

The narrative delves into the complexities of the Genestella system, revealing how these individuals are both revered and exploited for their abilities. Ayato's journey intertwines with the larger conflicts within Rikka, as he uncovers secrets about the Festa, the Foundation, and the true nature of his sealed power. The series balances action-packed battles with moments of character development, highlighting the relationships and motivations that drive the protagonists forward. Through its blend of competitive tournaments, personal quests, and overarching mysteries, "The Asterisk War" presents a story of determination and camaraderie in a world where power and ambition collide.