From the Godzilla Anime website, here is the synopsis as well as official posters for the third anime film (translated per the website).
The human race is on the brink of oblivion. The "wings of its demise" descend before its eyes.
This is the concluding chapter of the animated feature film series, GODZILLA (aka "Anigoji").
The final chapter, GODZILLA: The Planet Eater, finds the Mechagodzilla city, the peak of scientific evolution and mankind's best hope, 20,000 years into the future, the Earth is ruled by Godzilla. burned to cinders. Godzilla Earth reigns supreme but he has one more challenge: the winged creature, KING GHIDORAH.
The Godzilla franchise entered new territory in 2017 with Godzilla: Planet of the Monsters. As the first 3DCG animated, feature-length Godzilla movie ever, it offered a visual experience that was met with widespread acclaim and wonder when it debuted in November, 2017. The second chapter, GODZILLA: City on the Edge of Battle featured a newly-interpreted Mechagodzilla that only an animated rendering could accomplish.
With the concluding chapter of the saga, the King of the Void (GHIDORAH) squares off against the King of Earth (GODZILLA). When the dust settles, will there be anything left for the human race?
Directing the film are Kobun Shizuno, who last year earned the highest grossing Japanese movie to date with Detective Conan: The Crimson Love Letter ('17) and Hiroyuki Seshita who has burnished his reputation for gripping animated storytelling with such TV and film franchises as Ajin ('15 -16), Knights of Sidonia: Battle for Planet 9 (' 15) and Blame! ('17). Together this prodigious duo have forged a 3DCG genre of cutting edge expression. And now they bring all their talent to bear The story and screenplay was written by Gen Urobuchi, famously known for his Puella Magi Madoka Magica film series ('11 -' 14) and the 2012 animated movie, Psycho-Pass ('12). Urobuchi has a gift for creating mind-bending, innovative ideas that are sure to surprise and surpass audience expectations in this finale.
A door opens and a golden seal shatters a star.
But the search ends in vain, forcing them and their alien allies back to Earth. But 20 thousand years have passed in their absence, and the Earth is a wholly different place.
The planet's flora and fauna now embody and serve Godzilla. Earth is a monster's planet, ruled by the largest Godzilla ever at 300 meters in height.
The Houtua twin sisters, Maina and Miana, lead him to the skeletal remains of the Mekagodzilla, Taken from the Mechagodzilla carcass, the nanometals have gradually rebuilding a "Mechagodzilla City," a potential weapon capable of destroying Godzilla Earth.
As the strategy develops, a rift forms between the humans and the Bilusaludo, one of several alien races that had joined the humans on their exodus from Earth. Their leader, Galu-gu, believes that the secret to defeating Godzilla lies in the use of Haruo ultimately uses his means for defeating Godzilla Earth to destroy the Mechagodzilla city so as to prevent nanometal assimilation, killing Galu - superhuman powers - but, Haruo resists, fearing that in defeating monsters, they must not become monsters themselves. The childhood friend, Yuko, has been absorbed by the nanometal integration and has fallen into a brain dead coma.
The human race, once again, is lost. Metphies, commander of the priestly alien race, Exif, marvels at the miraculous survival of Haruo, he begins to attract a following. The Exif has secretly harbored this outcome as their "ultimate goal." Miana and Maina issue warnings against Metphies, while Haruo begins to question mankind's next move.
With no means for defeating Godzilla Earth, mankind watches as King Ghidorah, clad in a golden light, descends on the planet. The earth shakes once again with as war moves to a higher dimension.
What is Godzilla exactly? Does mankind stand a chance? Is there a future vision in Haruo's eyes? Find out in the finale.