A giant, fire-spitting turtle-like monster arises out of the Japan Sea after an atomic blast and goes on to destroy major Japanese cities until an international consortium of militaries is able to blast it out of earth in a rocket. Japanese cities

An atomic bomb explodes over the Arctic, angering Gammera, a huge, fire-eating, flying turtle aroused from its prehistoric sleep. As Gammera ravages entire cities, U. S. Air Force Gen. Terry Arnold, under orders from the Secretary of Defense and the President of the United States, consults with two Japanese scientists, Drs. Hidaka and Murase, but he is informed that the monster is indestructible by modern weaponry. U. S. Senator Billings opposes Arnold's request for emergency powers, and the controversy spreads to the United Nations Security Council. General Arnold, after receiving world cooperation, utilizes Plan Z, presented by Japanese scientists, as mankind's last hope. Gammera is distracted from its destruction of Tokyo by a volcanic eruption on the nearby island of Oshima and lands in the scientists' trap. They imprison the monster in a space rocket and shoot it to Mars.