In 2032, Noriko and Kazumi return to Earth. For them they've only spent four months in space, but because of the relativistic effects of near light speed travel, ten years have passed on Earth. They receive their diplomas from the Okinawa Girls' School, where their old rival Kashiwara has gone on to become an instructor. While leaving, Noriko encounters her old school friend Kimiko who now has a three year old daughter Takami.
In the evening, Noriko and Kimiko get to know one another again at a small playground. The outlook for humanity is grim. The Eltreum, an enormous 70 kilometer long ship, is being built in orbit in preparation for the evacuation of the planet. Since Noriko is in the military, Kimiko asks her to use connections to make sure that Takami gets a space. Noriko is "Takami's future." After leaving Kimiko, Noriko faces a sad and lonely evening on a world different than the one she left.
A month later at the Palomar observatory, an alien fleet of unimaginable proportions is detected. The number of aliens is so great, it almost can't be described as a fleet, measuring more than a hundred million in number and occupying the volume of a solar system.
Jung-Freud and Noriko are enjoying the sun at the beach. Jung tells Noriko that she's going to forget about Coachie and move on to new loves while there's time. Later in the day, Noriko reflects on these words and the revelation of the immense alien force. She thinks back to the events of the previous day when she was summoned to speak with Coach Ota. As she arrived, she caught the end of an argument between Kazumi and Ota. As Noriko stepped into the room, Kazumi rushed out on the verge of tears. Ota reveals that it is up to Noriko and Kazumi to save humanity with the assistance of Gunbuster. Years after the first encounter with the aliens, he still feels responsible for the death of Noriko's father. Helping Noriko save humanity seems to be the only way he'll find his own salvation. As Ota becomes increasingly ill, Noriko attempts to bring help. He prevents her, wanting to keep the truth hidden from Kazumi so she'll still be able to fight with a clear mind.
At a special meeting of the Earth's government, leaders argue about possible defenses but all seems hopeless as the earth is undermanned. Coach Ota barges in with a plan that might work: detonate the Exelion's warp core and create a man made black hole. The black hole will suck in the entire alien force. The plan is a longshot, but it is all the Earth has.
Noriko and Kazumi travel to Osaka where the two Buster Machines await. While the launch clock ticks down, Noriko makes idle conversation about he possibility of Kazumi and Ota getting married. Moments before the vehicles launch, Kazumi reveals that she knows he has space-radiation sickness and may be dead by the time they complete their six month mission.
Once in orbit, the two Buster Machines and the Exelion rendezvous and blast off, their mission becoming a race against time not only for humanity but especially for Coachie. In the few perceived minutes that it takes them to reach the target planet Raioh, four months pass back on Earth.
The trio of ships reaches the edge of the alien armada and begin the assault to reach the core of the aliens. Noriko and Kazumi quickly cut through the enemy, destroying aliens by the thousands. Unfortunately, as they near the center, Kazumi begins to break down. With six months passed on Earth, she sobs over all that she could've done with Coachie but didn't do. Having come so far, Noriko takes charge of the mission, talking Kazumi back in to composure. The fate of humanity is at stake. Now is the time to "fight!"
Emotionally recharged, they join forces and form the single Gunbuster unit. Using their combined strength, they're invincible, easily clearing the path to the heart of the enemy force. As ever more powerful enemies attack, they draw upon even greater weapons to dispel the alien menace.
Finally they reach the heart of the alien force and detonate the Exelion's core, forming a massive black hole. Virtually all of the aliens are destroyed. Mission accomplished, Gunbuster returns to Earth, landing directly at the hospital where Coachie was staying. Noriko and Kazumi rush in to find Coachie alive and well. Noriko goes back outside to leave Kazumi and Ota to finally express their true feelings for each other.
The earth is safe and all is well, for the time being.
To be concluded...