The ship crew of Harekaze II welcomes students from Kure, Maizuru, and Sasebo maritime high schools for the upcoming inter-high tournament in Yokosuka, setting a tone of friendly competition and teamwork. After exploring a variety of stalls at the welcome festival, Akeno Misaki and Mashiro Munetani form a quick bond with a foreign guest, Susan Reyes, whose presence adds a new dynamic to the group and raises questions about loyalties and the future. In the background, instructor Kaoru Furushou contends with the crew’s recommendation, pushing for Mashiro to be promoted to captain, which sows tension between the two mentors and the young sailors as they weigh tradition against ambition.
The next day brings a crisis that tests everyone’s resolve: Blue Mermaids receive a startling report that pirates hijack a floating water purification plant and a nearby sea fortress, intending to convert them into a mobile base for their operations. Concurrently, Susan infiltrates a massive float slated for scrapping, aiming to block the Blue Mermaids from leaving the harbor and forcing a confrontation that could alter the balance of control in the area. The looming threat throws the schools into a coordinated response, as the students and their supporters prepare for a high-stakes mission across water and fortress walls.
In a hospital, Susan reveals she was coerced by the pirates in exchange for help finding her father in Japan, a revelation that deepens the stakes for everyone involved. Responding to this plea, [Akeno Misaki], [Mashiro Munetani], and [Moeka China] join the Blue Mermaids and other girls’ maritime high schools in a concerted effort to prevent the two floating assets from rendezvousing with the pirates’ plans. The rescue effort accelerates as Benten and Admiral Graf Spee coordinate to free hostages and capture pirates, demonstrating the teamwork and resourcefulness of the ensemble. The Blue Mermaids ally with the all-male White Dolphins in a combined assault on the fortress, pushing back against heavy fire as the vessels exchange blows.
When the fortress returns fire, Akeno decides to infiltrate the structure, leveraging Harekaze II’s size to slip through a narrow breach opened by the high school fleet’s bombardment of four Yamato-class flagships. With Susan’s intimate knowledge of the fortress interior, the Blue Mermaids and their allies execute a decisive disablement of the fortress, turning a dire situation into a turning point for the mission. In the aftermath, Mashiro shares her heartfelt wish to remain with the crew, seeking to gain more experience while continuing to pursue her dream of becoming a captain, a goal that now feels within reach thanks to the lessons learned and the bonds formed during this perilous operation.
