Extra-terrestrial life known as Trench Walkers reveal themselves, emerging from the Antarctic, and walking on the sea floor. Their goal is to terraform the Earth to their liking, which involves raising the sea level by 70 meters, thus rendering Tokyo and many other population centers uninhabitable. They have also begun to change Earth's surface water into what they call Nectar, the "divine water." Those who are incompatible with Nectar have a high chance of death when imbibing it. However, there are girls who are found to be compatible with Nectar, and they are recruited to be marines in a fight against the Trench Walkers, boarding ships to do battle in a new deep sea front line. Hiro Todoroki, having received a message from her deceased sister, searches for the strongest Marine, Kiriri Kakuba, and, despite her doubts, decides to join the Masakado Girls' Academy Submarine Fleet—!?