In the near future, a hypercane has appeared in the Pacific Ocean, and it's been frozen in the same location for over 50 years. This causes all sorts of environmental problems due to the destabilizing effects caused by the storm. Due to the storm the world has revert to a second naval age. This has caused a rise in piracy, so many large international companies have begun arming themselves and setting up their own security forces. Hyōsuke Magamiya runs into the crew of the Pascal Mage, a military ship. He gets into a few typical compromising situations before finally being introduced as a software engineer assigned to update the systems onboard ship. He meet his step-sister, the captain of the war vessel, Captain Misaki Nanaha, who treats him as a stranger. Although he calls her sister he still thinks of her as a women none the less. During the duration of his duties, the ship is called into combat, where Hyosuke begins to understand the nature of the Pascal Mage and its crew's battle potential as the vessel flings itself into the fight.