A Japanese teen named Yamato Hino, a young sports enthusiast, during a quiet and ordinary day begins to feel strange calls from an indefinite dimension. Frightened and confused, the boy thinks that he had hallucinations but during a thunderstorm a lightning bolt drags him into a parallel world: the ancient kingdom of Mu, ruled by King Muraji whose concern is to defend the capital of his kingdom, attacked by monsters commanded by the evil Dorado of the Empire of Dinosaurs.
In the kingdom of Mu, a legend tells of a titanic being that stands ready to defend the population from any threat and the king wants to awaken the giant statue of the god Mazinger; to do this he needs a brave boy whose name is Yamato, the only one capable of awakening the mighty giant. The devout prayers of the king and the princess are heard: the boy arrives to the dimension of Mu and the statue suddenly becomes alive. A luminous beam covers the boy that is absorbed into the body of the statue, which begins to move. When the monsters of Dorado broke through the defenses of the kingdom, God Mazinger defeats the dinosaurs and makes the enemy army flee. And thus the legend becomes a reality.
Yamato joins the court of newly crowned Queen Aira Mu, becoming the champion of the Kingdom of Mu, always ready to repel the attacks of the evil Dorado and his fearsome dinosaurs.