The trailers sell you on cute girls in sailor uniforms. Episode two introduces a biological weapon and a military conspiracy that forces real tactical decisions with real casualties. High School Fleet looks like comfort food but operates like a war thriller that actually respects the naval combat enough to make every engagement feel like it matters.

A century after shifting tectonic plates sank most of its land, Japan has become a prosperous maritime nation, with busy coastal cities and sea lanes guarded by dedicated naval vessels. Many girls dream of joining the Blue Mermaids, skilled women who protect the ocean. Childhood friends Akeno Misaki and Moeka China meet again at Yokosuka Girls' Marine High School after a period apart, surrounded by classmates who hope to become Blue Mermaids too. When training begins, each of them is given a ship and crew of her own. Akeno settles in as captain of the destroyer Harekaze and grows close to her crewmates, until sudden mechanical and navigation problems leave the ship stranded and its crew at risk. The mix of action and comedy follows the Harekaze's crew through dangerous waters as they work toward becoming full Blue Mermaids.