 A freighter is attacked by a giant octopus during a stormy night. The octopus is then attacked buy a hairy green giant. The gargantua then turns his attention to the ship and sinks it killing and eating all the crew except for one. When the surviving crew member tells the police what happened, they call in Doctor Paul Stewart and his assistant Akemi. Several years earlier, Stewart had a younger species of the same creature and it is feared that the monster has grown up and now views man as an enemy. After several attacks, the monster is cornered in a forest in a mountain area of Japan, and is nearly killed when shot with lasers and nearly electrocuted. Just before it dies a second, bigger brown monster arrives and saves it. This is the creature that Stewart had at his lab now all grown up to over 100 feet tall. The brown one then takes the green monster away to help it recover, not knowing his "brother s" evil nature. After the green monster recovers sufficiently, he resumes his rampage. But when he stops to rest, the brown one finds out how evil his brother is and soon the two begin to fight. The green one then runs off and soon heads for Tokyo. While he is there, he grabs Akemi and is about to eat her when the brown one arrives. The green monster drops Akemi, severely injuring her. The two monsters then resume their fight and eventually wind up in Tokyo Bay where napalm bombs are dropped on them, seemingly killing them.