Godzilla washes ashore and is almost dead because of diabetes. The Japanese Government starts dissecting Godzilla apart and discover that she's pregnant and her removed-brain is impossibly receiving a transmission from a Dark Nebula.
Still inside the womb, Baby Godzilla psychically bonds with a girl named Momo.
The Japanese Government calls for a psychic to communicate with Godzilla's Brain, which reveals that her name is "Rozan" and she comes from "Planet Godzilla". There is major politic discussion on how to deal with Rozan and Baby Godzilla, and Momo singing on national-television convinces them to send the Godzillas back to their planet.
Except Rozan is fucking huge and building a rocketship for her is infeasible. So they instead turn her into a space-rocket (literally the kind that breaks into three pieces), with the womb attached outside to her chest.
Once past Jupiter, Rozan starts warp-driving out of the Milky Way (it's mentioned later that a single warp can go as far as 500 million light-years in a single leap).
Rozan names Baby Godzilla as "Ririn".
During the journey, Rozan is slowly regenerating to her full body because the Godzilla species can recover from a single cell. She is attacked by "Rocks With Beaks" and she removes an arm for them to feed on; She reaches the "Desert Of The Universe" which is folding her like an origami until she breaks the space-time distortion by plucking out her eyeballs and using them as bombs; She almost loses Rinrin to an illusion induced by a "Black Cloak" from "Planet Manto".
After the last warp into the Dark Nebula, Rozan absorbs the womb back inside of her and naturally gives birth to Rinrin. And then an exhausted Rozan remains in orbits as she pushes Rinrin into the gravity of Planet Godzilla to meet his father.
Except, it turns out that Planet Godzilla has been occupied by another alien-race known as the "Sunerians" (An erotic species of females that look similar to Satyrs). Rinrin gets captured and brought to the Sunerian Queen who takes a look into the Baby Godzilla's memories. Seeing the images of Rozan attacking Japan, the Sunerians conclude that Humans must be an especially vicious race to have made a member of the saintly pacifist-race of Godzilla to go berserk. So the Sunerians decide that they should use the psychic-bond between Rinrin and Momo to send a "Wave Motion Virus" that would make all humans go crazy and kill each other (furthermore, Earth would be contaminated for 1000 years).
The Sunerians start torturing Rinrin. Momo becomes aware of this and calls out to Rozan to save her child. Rozan goes to the planet and attacks the Sunerian Palace. Meanwhile, Kuunin (the husband of Rozan and father of Rinrin) hears about this from some other Godzilla, and leaves his cave. But before he flies off, two Sphinxes (a slave race of the Sunerian who look as their namesake) come running with Rinrin. Eventually Rozan is burned down to nothing by the Sunerian's counterattack.
Kuunin rallies the other Godzillas to fight the Sunerians.
Meanwhile, the two Sphinxes, who are actually a Prince and Princess sneak back into the Palace to revolt. They start blowing up the place and the Prince is killed, while the Princess manages to scare the Sunerian Queen to retreat back into outer-space.
Kuunin fights against "General Gamoni" (who is less of a Satyr and more of a giant lizard-frog) and eventually tears off her head, ending the battle. The Sphinxes then board a pyramid-shape spaceship and leave for parts unknown.
At the end, Kuunin makes a comment how Rozan loved Earth but wonders why the Gods would create a race like the Humans who are contradictorily filled with intense Love and Hate. And says that Rinrin should go back and visit when he's older to find an answer.