A pirate fortress is bearing down on Japan, and four girls' schools are the only thing standing between it and the coast. The combat sequences are sharp enough to make you forget this is a spinoff, built on accurate naval choreography and sound design that makes every shell casing matter. Mashiro's choice between duty and loyalty plays out across real stakes, not dramatic nonsense, and the girls who crew these ships are competent enough that you believe them in combat, not just cute enough to watch.

Four marine academies converge for a dangerous mission: stop a pirate-controlled fortress armed with serious firepower from reaching Japan. The trouble begins while Yokosuka Girls' Marine High School is hosting other all-girls maritime schools for festivals and competitive games, until pirates seize a sea fortress and a water purification plant. But for Mashiro, the crisis arrives alongside a personal crossroads. Her instructor offers her the chance to lead the Hiei class, which would mean leaving the Harekaze and her friends, and both she and captain Akeno Misaki are left with mixed feelings. With the decision still pending, the two must command the Harekaze through the operation. The film pairs naval action with comedy and camaraderie.