In the far-off future, humanity's remnants seek safety in expansive citadels known as Plantations, having been nearly eradicated by colossal beasts called Klaxosaurs. These fortified cities serve as havens, where children are reared with the specific goal of piloting FranXX mechas – humankind's sole viable weaponry against the Klaxosaurs – with one girl and one boy operating the machines in tandem. These youngsters are completely unaware of the outside realm, save for fulfilling their mission to defend their species. Hiro, a hopeful FranXX pilot, becomes dispirited and doubts his own abilities upon failing his aptitude exam, causing him to boycott his class's graduation ceremony. Instead, he happens upon Zero Two, a girl with two horns recognized for her FranXX piloting skill and dubbed the "Partner Killer." As fate would have it, a Klaxosaur assault occurs on the Plantation they inhabit, resulting in Zero Two clashing with the creature in her FranXX. Her mecha is severely harmed during the encounter, and she loses her partner in the battle. There, she entreats Hiro to become her partner and together, they effortlessly repel the Klaxosaur. With a new partner, Hiro is presented an opportunity to redeem himself – but at what cost?