Sole Salvation

Young Zeke grows up inside Liberio's internment zone, doused with water by a janitor for stepping outside it and taught by Grisha that a caged world must be changed. His parents enroll him as a Warrior candidate purely to plant a Restorationist inside Marley's Titan program, drilling forbidden history into him at the expense of any real affection. He trails the other candidates in training, and the arguments he overhears at home make clear that his worth to Grisha and Dina is strategic.

Everything shifts when a stray baseball rolls to his feet. Tom Xaver, the scholarly holder of the Beast Titan, discovers the boy's gift for throwing and becomes the first adult to simply spend time with him. When Marleyan security closes in on the Restorationists, Xaver advises the frightened child to denounce his own parents, and Zeke does, earning acceptance from his peers while Grisha and Dina vanish to Paradis. Years later a dying Xaver shares his research: the Founding Titan can rewrite the bodies of the Subjects of Ymir. Zeke extends the idea into a plan, sterilize them all and let the race fade within a century, ending both the Titans and Eldian suffering. He inherits the Beast Titan by eating his mentor, dons his glasses, and calls him Father.

Learning from Reiner and Bertholdt that the Founder rests with Eren, Zeke concludes Grisha repeated his indoctrination on a second son and resolves to save him. In Marley, Eren appears to embrace the euthanasia plan. The recollection closes in the present-day cart, where Zeke screams Xaver's name, wrenches his head back, and triggers the Thunder Spear buried in his stomach, tearing himself in half and hurling Levi into the air.

Zeke's childhood is revealed: raised as a Restorationist tool inside Liberio, he lags in Warrior training and finds no warmth at home.

Tom Xaver befriends him over games of catch and later prompts Zeke to report his parents, sparing him their fate.

The euthanasia plan is born from Xaver's discovery that the Founding Titan can alter Eldian bodies; Zeke pledges to take on the Beast Titan and end the race painlessly.

Zeke pledges himself to Eren, believing his half-brother is a fellow victim of Grisha, and Eren appears to agree to the plan in Marley.

The Thunder Spear detonates: back in the present, Zeke sets off the spear in his abdomen, blasting Levi and bisecting himself.

Sole Salvation is the 15th episode of season 4 and the 74th overall, a MAPPA production directed by Mitsue Yamazaki and Yoho Ishikawa from Hiroshi Seko's script. It aired March 22, 2021 in Japan and April 25, 2021 in the US, adapting the chapters Sole Salvation and Support.

The episode also explains the loophole around King Fritz's renunciation of war: royal blood touching the Founder's holder can unlock its power, though the choice stays with whoever holds the Founding Titan. Xaver frames Zeke as the key, not the lock.