In the distant future, humanity has been driven to near-extinction by giant beasts known as Klaxosaurs, forcing the surviving humans to take refuge in massive fortress cities called Plantations. Children raised here are trained to pilot giant mechas known as FranXX—the only weapons known to be effective against the Klaxosaurs—in boy-girl pairs. Bred for the sole purpose of piloting these machines, these children know nothing of the outside world and are only able to prove their existence by defending 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, Hiro retreats to a forest lake, where he encounters a mysterious girl with two horns growing out of her head. She introduces herself by her codename Zero Two, which is known to belong to an infamous FranXX pilot known as the "Partner Killer." Before Hiro can digest the encounter, the Plantation is rocked by a sudden Klaxosaur attack. Zero Two engages the creature in her FranXX, but it is heavily damaged in the skirmish and crashes near Hiro. Finding her partner dead, Zero Two invites Hiro to pilot the mecha with her, and the duo easily defeats 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?

There is a tower that summons chosen people called "Regulars" with the promise of granting their deepest desires. Whether it be wealth, fame, authority, or something that surpasses them all—everything awaits those who reach the top. Twenty-Fifth Bam is a boy who had only known a dark cave, a dirty cloth, and an unreachable light his entire life. So when a girl named Rachel came to him through the light, his entire world changed. Becoming close friends with Rachel, he learned various things about the outside world from her. But when Rachel says she must leave him to climb the Tower, his world shatters around him. Vowing to follow after her no matter what it takes, he sets his sight on the tower, and a miracle occurs. Thus begins the journey of Bam, a young boy who was not chosen by the Tower but opened its gates by himself. They call his kind "Irregulars"—beings that have shaken the very foundation of the Tower each time they set foot inside it.

Fleeing a traumatic childhood, lone mercenary Guts drifts from battle to battle, striking down powerful enemies with his unwieldy sword. After one skirmish, Guts clashes with the mystifying Griffith and his small army known as the Band of the Hawk. The victorious Griffith drafts Guts into his ranks, granting the swordsman a kinship unlike anything he has ever known. Alongside the skilled warrior Casca, Guts helps Griffith guide the band to fame and glory. But as he gains military prestige, Griffith becomes the target of assassination attempts and political schemes. He uses Guts to further his ambitious rise to power, taking advantage of the swordsman's lack of aspiration. Even as he grows closer to Casca, Guts' absence of direction robs him of any sense of belonging. Fostering opposing desires, Guts and Casca fight for Griffith's dream—although fractures forming among the trio threaten to disrupt their world.