Movie Story: Ten years after the defeat of the underground empire of Mikene, the Earth and all its inhabitants have finally found peace, thanks also to photonic energy, a clean and practically infinite source of energy.
During some excavations on Mount Fuji, a gigantic head belonging to a colossal Mech Mazin (later called Mazinga Infinity, or Infinite Demonic Deity) is found, Koji Kabuto, now a researcher on photonic energy, is called to investigate the presence of a form of life within the giant. The young man approaches the giant robot from which a girl comes out, later called Lisa by Koji himself.
A few months later, a Texas photonic energy center is attacked by some reviving mechanical monsters that put a strain on the base defenders, latter, however, are saved by the providential intervention of the legendary Tetsuya Tsurugi, who takes the field at the wheel of his Great Mazinga, with whom he manages to "scrape" many assailants, but suddenly the evil Baron Ashura appears at the head of his robot and an explosion makes it impossible to see the outcome of the clash between the two.
Meanwhile, the beautiful Jun Hono, now wife of Testsuya and pregnant, goes to visit Koji and Sayaka, now head of the photonic energy research center on the structure erected on Mount Fuji, where the excavations have unearthed the bust of the gigantic Infinity (along with the Mycenaean ruins), studied by Koji and the various researchers who also realized that Lisa, actually an android (even if she defines herself mainly organic), is the key to activate the metal giant, while Koji theorizes that the increase in photonic energy has teleported her, the robot and the ruins, from another dimension.
While the news of the attack in Texas and the disappearance of Tetsuya arrives, the center in which Koji and his companions lie is besieged by the army of iron crosses led by the evil Count Blocken, who forces our heroes to escape, succeeded thanks to Lisa.
The Japanese government sends the armed forces, with the moral support of Koji and with Shiro (his pestiferous, but nice younger brother) at the head of the team of assault of mech similar to Mazinga to regain the base where the Infinity was located, but the attack ends in a disaster when a colossal mech, now active, sweeps away the occupying forces from a strange portal, among the few survivors there are Shiro and Koji, meanwhile arrived on the spot, who realizes that the Great Mazinga is imprisoned on the head of the Infinity, with Tetsuya inside it alive, but unconscious.
The evil Dr. Inferno, the nemesis of Mazinga Z, reappears, announcing to the world that he wants to live peacefully with the human race, provided he is given a certain amount of photonic energy, otherwise the planet will be completely destroyed, the news rekindles the discord between the various governments of the world, which are divided between those who want to comply with the demands of Dr. Hell and who, however, wants to stop it.
Shortly thereafter, Koji and Lisa visit Boss, who has since opened a Ramen shop, and then try to understand what Dr. Hell really has in mind. At this juncture, Lisa tells Koji what was told to her by the Infinity, that anyone who controls the Mech can activate the "Goragon" (strange word pronounced by the girl when she was just found), a power that whoever considers it necessary can destroy the universe and replace it with one of its own choosing, since the mech is nothing more than a weapon to reshape the cosmos, in addition, activation is now close. Koji reveals to Lisa the last words of his dying grandfather: "Who drives Mazinga Z can become a god or a demon".
Koji understands that Dr. Hell has kidnapped Tetsuya to use him and the Great Mazinger as a feeder for Infinity, without having to use Lisa as an activation key. The true purpose of the mad scientist is not to control the world, but to study it and, once tired, to recreate it at will.
Koji devises a plan to stop Infinity and save Tetsuya thanks to his companions and various scientists at the old photo-atomic center where Mazinga Z is kept, with which Koji intends to face the army of mechanical monsters after repairing the robot, in addition, Lisa will accompany him to stop Infinity. As soon as the news spreads, humanity reacquires hope. While Boss Robot and a car piloted by professors Sewashi and Nossori occupy most of the enemy forces, thanks to the intervention of the Mazin-Girls, a group of female pilots driving a team of Venus Ace (the robot that once was driven by Jun), Mazinga Z secretly enters a tunnel, then opens the way to the summit of Infinity with a series of spectacular fights.
While he is intent on freeing Tetsuya from his captivity, Koji is attacked by the Great Marshal of the Demon, or the personal robot of Dr. Hell, with whom he engages in a furious battle. Being severely damaged, the scientist reveals to the hero that this peace has softened him and that the boy's true nature is war, then he asks Lisa something in a strange language. Fortunately, Shiro, aboard his mech, frees the Great Mazinga and both join Mazinga Z in the fight, managing to damage the Great Marshal of the Devil, but Doctor Inferno replaces him to the Great Mazinger (having copied this data last on his robot) as an energy source for Infinity, activating the mech and starting "Goragon".
The situation seems desperate, but Lisa, after asking Koji if the world deserves to exist (the strange words spoken by Dr. Hell) and the boy answers yes, finds a way to give Mazinga Z similar powers to those of Infinity, regenerating it and making it as gigantic as the latter, not being able to bypass the control of Dr. Hell on the mech, even with the new powers, however, the Koji robot seems not to be able to stand comparison with Infinity, at least until Lisa sacrifices himself by conveying the photonic energy of the Great Mazinga, Shiro's mech and all that present on the Earth in the body of Mazinga Z that with a very powerful rocket punch sends Infinity into space and makes it explode, bringing peace back to Earth.
Koji wakes up in a hospital surrounded by friends and asks Sayaka to marry him. In the final scenes, we witness a wedding from a distance and, later, we see a girl with her parents walking near Mazinga Z.