The film is commemorating the 45th anniversary of the original Mazinger Z series.
It has been 10 years since then. Humanity awaits a new fate.
Once humanity was on the verge of destruction at the hands of the evil scientist Dr. Hell and his underground empire. Kouji Kabuto, his Mazinger Z super robot, and his friends at the photon power laboratory put an end to Dr. Hell's evil ambitions, and returned peace to the world.
It has been 10 years since then. Kouji has followed his father and grandfather and now walks the path of a scientist. He encounters a mysterious life form buried in a giant structure buried deep below Mt. Fuji. A new encounter, a new threat, and a new fate await humanity. And Kouji, who was once a hero, makes a decision for the future.
It can be a god or a demon.
Once again the future is entrusted to Mazinger Z.
(Source: Anime News Network)
— Description from AniList community (anilist.co).
