Admiring female Marines known as the Blue Mermaids since childhood, Akeno Misaki enrolls at Yokosuka Girls' Marine High School in order to become one herself. After briefly reuniting with her childhood friend Moeka China, who is assigned as captain on the ship Musashi, Akeno is assigned as captain on the destroyer ship Harekaze, much to the ire of Deputy Captain Mashiro Munetani. As the ships head off towards nautical lessons, the Harekaze, which is running late due to engine troubles, suddenly comes under attack from the instructor's ship, the Sarushima. After narrowly evading attacks with no response from the instructor, Akeno makes the decision to launch a dummy torpedo at the Sarushima so they can safely retreat. However, this leads to the Sarushima reporting that the Harekaze has mutinied.