"The Asterisk War: The Academy City on the Water" is set in a futuristic world where humanity has developed advanced technology and genetically enhanced 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 academy is one of six institutions that participate in the Festa, a series of competitive battles where students, known as "Starpulse Generation," showcase their combat skills and abilities.

The protagonist, Ayato Amagiri, transfers to Seidoukan Academy to search for his missing sister, Haruka, who disappeared under mysterious circumstances. Upon arrival, Ayato inadvertently becomes involved in a duel with Julis-Alexia von Riessfeld, a skilled duelist and princess of a small European nation. Their encounter leads to Ayato accidentally damaging Julis' prized garden, and as a result, he is forced to partner with her in the Festa to repay his debt.

As the story progresses, Ayato and Julis form a partnership and compete in the Festa, facing off against other powerful students from rival academies. Ayato wields a unique weapon called the Ser-Veresta, a sword with a sealed power that he gradually learns to control. Julis, on the other hand, specializes in fire-based abilities and wields a rapier. Together, they navigate the challenges of the Festa while uncovering secrets about the Genestella, the Festa's true purpose, and the shadowy organizations manipulating events behind the scenes.

Other key characters include Claudia Enfield, the student council president who aids Ayato and Julis, and Saya Sasamiya, Ayato's childhood friend and a skilled sniper who enrolls at Seidoukan to support him. The series explores themes of rivalry, camaraderie, and the pursuit of personal goals, as the characters confront their pasts and strive to achieve their ambitions within the competitive and often dangerous world of the Festa.

The narrative balances action-packed battles with moments of character development, as Ayato and his allies grow stronger and forge deeper bonds. The series also delves into the political and social implications of the Genestella's existence, as well as the ethical dilemmas surrounding their enhanced abilities. Through its blend of high-stakes competition, interpersonal dynamics, and underlying mysteries, "The Asterisk War: The Academy City on the Water" presents a multifaceted story set in a richly detailed world.