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.

(Source: ANN)

A movie recapping the events of the Xabungle TV series.

Fourteen-year-old Kouichi Hayase's life has always been a mediocre one, if not dismal. However, those days of being bullied by classmates and escaping to a fantasy of being a hero are put to an end when a certain "accident" bestows on him a girl and a gigantic humanoid robot called "LINEBARREL." The extraordinary power that Kouichi obtains puts him and everything around him on a sudden rollercoaster ride of battles, intrigues and friendship! During the course of his adventure, the boy starts to learn what life has to offer; he meets new friends, bids farewell to the old ones, but most of all he now has considerable responsibilities and is forced to confront the world around him. (Source: Official site)

Episodes 25 & 26, added to DVD.