PKN was established to keep the ocean peaceful by bringing together advanced battleships from various nations. However, during their inaugural ceremony, the USR organization attacked with Admiral Red's UX submarine in a bid for world domination. Thanks to the bravery of Captain Hayami and his old Japanese submarine, the 707, they managed to rescue the others from being overpowered. A year later, PKN launched their newly constructed 707 II submarine with trainees Kenji, Goro, and Senta as part of the crew. They assembled a fleet, led by the latest US battleship, Great Guardian, but were unable to sustain the brutal blows from Submarine U, which led to their defeat. Just then, the 707 appeared, and Red was delighted to reunite with his rival and engage in battle with him. As they attempted to dodge Submarine U's attack, they stumbled upon a valley, which provided them with refuge. Red, however, attempted to destroy the valley to cause a flood that would drown the 707 in muddy torrents. They narrowly escaped and were caught in a pure water layer, causing the submarine to sink. Torpedoes were suddenly launched their way.