In the distant future, the earth once became a frozen planet. The surviving humans immigrated to outer space, established a prosperous colony planet Avalon, and established the Human Galactic Alliance with aliens. At the same time, the alien named Hidiers raged and multiplied, and launched a protracted war with the alliance. In a battle against the home planet of Hidiers, the 16-year-old Lieutenant Red (voiced by Kaito Ishikawa) encountered fierce resistance. During the retreat, Red was accidentally involved in a space-time rift, and fell into the ocean of the earth with the annihilation weapon Chamber he drove. Chamber was salvaged by humans to the Gargantia fleet. Faced with this human origin star that was completely different from his impression, Red was at a loss. During this period, he communicated with the lovely girl Amy (voiced by Hisako Kanemoto), and successfully helped them thwart the pirates' attack and won the trust of the earthlings. Unable to contact friendly forces, Red could only stay on Earth temporarily. He had only known how to fight and obey orders since he was a child. Facing this group of humans who had no discipline and efficiency, the deepest emotions in his heart slowly awakened...