The 2nd PriPara film, unlike the first this shown in stereoscopic 3D.

There is Earth, our familiar world, and then there is the Black World, a parallel dimension that very few people are aware of. For centuries, a pact between the two worlds has been observed to maintain peace, and terms must be negotiated and renewed soon to continue relative harmony. This time around, there is a militant faction that will stop at nothing to prevent the signing of a new treaty for inter-dimensional peace. Two agents of the elite organization known as the Black guards—defenders of the balance between the two worlds—are charged with ensuring the success of the treaty. Director Yoshiaki Kawajiri (Ninja Scroll) blends stylish eroticism, graphic horror and pulse-pounding action as these two race to consummate the peace treaty in time. (Source: AniDB)

There's a crisis in the Chinese Spirit World -- humans don't believe in gods anymore! A Door God, facing unemployment, ventures into the human world to prove his worth, leading to unexpected encounters and transformations for humans and spirits alike.

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)