To maintain peace in the ocean, the PKN was established. It comprised state-of-the-art battleships from various nations. However, during its founding ceremony, it was raided by the unidentified organization USR, which plotted to conquer the world. The PKN forces were no match for the submarine UX, operated by USR's Admiral Red. But an old Japanese submarine, the 707, saved them; it was piloted by Captain Hayami, who risked his life. One year later, succeeding the 707, which had been destroyed in the previous battle, the newly developed submarine 707 II set sail. Among the crew were boys such as Kenji, Goro, and Senta, who had been trainees. The PKN formed a fleet led by the Great Guardian, the latest battleship of the United States. However, the attacks by Submarine U were very severe, and the fleet was defeated. Then, the 707 arrived. As soon as Red noticed the 707, he was glad to reunite with his rival and began to fight. Escaping the submarine attack, they hid the 707 in a valley. But Red tried to destroy the valley in order to bury the 707 in torrents of mud. The 707 barely escaped and ended up in a crater. However, it was a layer of pure water where the submarine could not float up. At that moment, numerous torpedoes were launched at the 707.