In a far future ravaged by giant creatures called Klaxosaurs, humanity lives within fortress cities. Children are raised as pilots for giant mechs called FranXX, the only weapons effective against the monsters. These pilots, paired as boy and girl, are trained to fight, knowing nothing else.
Hiro, once a promising pilot, has lost his way after failing an important test. He meets Zero Two, a mysterious and dangerous girl with horns and a reputation as a "Partner Killer."
When a Klaxosaur attacks, Zero Two needs a new partner to pilot her FranXX. Hiro steps up, and together they successfully defeat the creature. This gives Hiro a second chance, but partnering with Zero Two comes with unknown risks.