In the distant future, humanity is on the brink of extinction due to giant beasts called Klaxosaurs. To survive, humans have taken refuge in massive fortress cities known as Plantations. Children born here are trained from a young age to pilot giant mechas known as FranXX-the only weapons capable of defeating the Klaxosaurs. These children know nothing about the outside world and their existence is solely defined by their ability to defend their race.
Hiro, an aspiring FranXX pilot, has lost his motivation and self-confidence after failing an aptitude test. Skipping out on his class graduation ceremony, he retreats to a forest lake where he encounters a mysterious girl with two horns growing out of her head. She introduces herself as Zero Two, an infamous FranXX pilot known for being the "Partner Killer." Before Hiro can process this encounter, the Plantation is attacked by a Klaxosaur. Zero Two engages the creature in battle using her FranXX but it is heavily damaged and crashes near Hiro. Finding her partner dead, Zero Two invites Hiro to pilot the mecha with her, and they easily defeat the Klaxosaur in the ensuing fight. With a new partner by his side, Hiro has been given a chance at redemption for his past failures but at what cost?