In an underground ISO city in Siberia that uses mantel energy, a huge explosion takes place which results in a high level of radioactivity. Causing this has been Megatonra, a monster which has been brought from space by Governor X and which grows by absorbing radioactivity. Megatonra has been put under the Earth by Gel Sadra where it has been eating radioactive waste and has grown considerably. Then it has started to destroy the underground city. Dr. Horin is the husband of Mary, the architect of the city, that died in an accident. Ken goes with him to investigate the underground city. But Mark, the son of Dr. Horin, has secretly sneaked in the expedition car. Mark hates the underground city because it took away his mother and he intends to destroy it. The monster appears and destroys the Mark Park, which Mary left behind for her son. When Mark sees this, he flies away in the expedition car in order to protect the underground city constructed by his mother.
