By 2445, the ocean itself has become enemy territory. A fleet of hostile extraterrestrial beings settled their ship on the Pacific Ocean floor and began raising sea levels — five years of continuous flooding until nearly ninety percent of Earth's surface disappeared beneath the water. The Earth Ocean Alliance's response was the Hydra: a single-pilot submarine fighter engineered for deep-water combat against an enemy that conventional navies cannot touch. With the other pilots gone, one remains. Armed with a Hydra and no backup, the last standing fighter descends into the alien-infested sea — mission by mission — to hold whatever territory is left and eventually take the fight to the source.