In the distant future, humanity is on the brink of extinction due to giant beasts called Klaxosaurs. Survivors live in fortress cities called Plantations, where children are trained to pilot mechas called FranXX to fight the creatures. Hiro, a failed pilot, meets a mysterious girl named Zero Two, a notorious FranXX pilot known as the "Partner Killer." After a Klaxosaur attack, Hiro teams up with Zero Two, and together, they defeat the beast. With a new partner and a chance at redemption, Hiro’s journey begins, but he must face the unknown cost of this new alliance and his past failures.