Across the oceans, a terrorist group known as the USR is indiscriminately attacking other ships. To combat this menace, eleven countries join forces to form the Peace Keeping Navy, or PKN. Among them is Japan, whose representative—the Submarine 707—is a plodding underdog compared to the technological marvels of countries like America. In fact, the 707 is so slow in getting to the PKN's first meeting that it inadvertently sacrifices itself when the USR makes a surprise attack. After a new submarine is built, the 707's Captain Hayami comes out of retirement and prepares to defend the seas again. One by one, the technologically superior USR picks off the other nations' vessels, until finally Hayami must lead the 707 into battle with sheer guts and skill.