To combat the terrorist organization known as "USR", eleven countries combine their naval forces to form the Peace-Keeping Navy (PKN). However, all hell breaks loose when Admiral Red and his USR submarine UX crash the PKN's inaugural ceremony and destroy majority of the fleet. At the last minute, when the UX lands the final blow on the aircraft carrier Apollo-Norm, the obsolete Japanese submarine 707 (a.k.a. "Snapping Turtle") appears from out of nowhere and sacrifices itself to save the PKN flagship. Six months later, Captain Youhei Hayami is called back to duty to lead a new group of recruits and captain an all-new 707 in the war against terrorism underwater.