Set in a future where rising sea levels have altered the world’s geography, "Hai-Furi" centers on the students of Yokosuka Girls’ Marine High School, a specialized institution training young women to become "Blue Mermaids"—skilled sailors responsible for protecting the seas. The protagonist, Akeno Misaki, enrolls in the school to follow in the footsteps of her late mother, a renowned Blue Mermaid. Akeno is assigned as the captain of the training ship Harekaze, alongside her childhood friend Moeka China, who serves as the ship’s navigator.

The narrative begins with a routine training exercise, but the situation quickly escalates when the Harekaze receives an emergency distress signal. The crew discovers that the Musashi, a state-of-the-art battleship, has gone rogue and is attacking other vessels. The Harekaze is mistakenly identified as an enemy ship and becomes a target, forcing the inexperienced crew to navigate a dangerous situation. As they evade pursuit, the students uncover a larger conspiracy involving the Musashi and its mysterious AI system, which appears to be malfunctioning.

Throughout the series, the crew of the Harekaze faces numerous challenges, including battles with other ships, resource shortages, and internal conflicts. Key characters include Mashiro Munetani, the ship’s gunnery officer, who struggles with self-doubt; Shima Tateishi, the chief engineer, who works tirelessly to keep the ship operational; and Rin Shiretoko, the communications officer, who provides crucial intelligence. The crew must rely on their training, teamwork, and ingenuity to survive and uncover the truth behind the Musashi’s actions.

The story balances action and character development, exploring themes of leadership, responsibility, and camaraderie. As the Harekaze’s crew grows closer, they learn to trust one another and adapt to the unpredictable nature of their mission. The series concludes with a climactic confrontation that resolves the immediate threat but leaves room for further exploration of the world and its mysteries.