In a post-apocalyptic world overrun by monstrous Klaxosaurs, humanity teeters on the brink of extinction, finding refuge within massive fortress Plantations. Trained from youth to pilot FranXX mechas, the only weapons against Klaxosaurs, children exist solely for combat, unaware of life beyond their duty. Hiro, a hopeful FranXX pilot, loses faith after a failed test, escaping to a forest where he meets Zero Two, a mysterious girl sporting horns and a notorious reputation as the 'Partner Killer.' Amidst turmoil, a Klaxosaur attack thrusts them together, leading Hiro to join Zero Two in battle. As they triumph over the enemy, Hiro gains a chance at redemption, yet the price remains uncertain.