That is an UFO! The Flying Saucer

Alien invaders take over Great Mazinger who is at the entrance of a museum next to Mazinger Z as symbols of peace. Grendizer fights the evil invaders and Great Mazinger is recovered. When the Vegan empire dispatches a new general to oversee the takeover of Earth, he quickly manages to capture Koji Kabuto and, learning of Great Mazinger through interrogation, quickly commandeers it from a museum where it was kept as a symbol of peace. It is now up to Duke Fleed aboard Grendizer to stop humanity's former defender from becoming its destroyer.

This is a regular episode of UFO Grendizer that was shown as a short movie in theaters.

Asaba Naoyuki is an ordinary high school student. As a member of his school's press club, he's just spent the summer camping outside the local military base, in hopes of seeing the UFOs that are secretly kept there, according to local legend. Returning to school, he meets a strange girl, Iriya Kana, and gradually comes to realize that she is more than merely strange - and that a dark secret lies beneath the world that he knows.

The story is about 16 years old boy Takeshi who recently formed an American football team at the BlueSky orphanage. One day the game is interuppted by a light in the sky. Takeshi discovers he is the son of the king of the planet Apolon, spirited away to Earth by his father's retainer Rabi to avoid death at the hands of General Dazaan. He turns out to have special energy abilities which can control UFO saucers from under the ocean floor, which in turn can become robots that are synchronized to their movements.