A giant sea monster known as "Dragonsaurus" has emerged from out of nowhere, terrorizing the depths of the oceans. Great Mazinger, Grendizer and the Getter Robo G team join forces to combat the new threat. But their task becomes more complicated when Dragonsaurus swallows up Boss Borot and makes its way towards Tokyo. 

Watch Grandizer, Getter Robot G, Great Mazinger Decisive Battle! The Great Sea Monster Online