Past Time

Across a year in captivity the Reaper's body keeps mutating while his bond with Aguri deepens. A test of the tentacle formula on a lab rat ends catastrophically, triggering the blast that erases most of the moon. Warned that the same fate awaits him in a year, the Reaper unleashes his new powers to break out of the facility, killing guards and destroying Yanagisawa's left eye in the escape.

As Aguri tries to stop him from leaving, a hidden trap fatally wounds her, and with her last words she asks him to look after Class 3-E in her place. Devastated, he wishes to become weak and flawed, and the tentacles reshape him into the gentle Korosensei. Back in the present, the class reels at the thought of assassinating the teacher they love, and Nagisa declares his intent to find some way to save Korosensei's life instead.

The March anniversary of the pair's first meeting becomes the day the crescent moon takes its permanent shape, a direct product of Yanagisawa's antimatter research. Faced with the risk that Earth could meet the moon's fate within a year, Yanagisawa moves to eliminate his subject, only for the transformation to enable the escape. Aguri's death and her dying request become the foundation of Korosensei's whole purpose, and Kayano's own resolve shifts once she learns the truth.

To match the somber tone, the episode drops the opening sequence entirely and shows only the title card, using the insert song Moonlight during the flashback. The anime cuts the self destruct device implanted in Korosensei, relocates his fight with Yanagisawa's men to a hallway, and alters details of the anti tentacle barrier and how Yanagisawa's eye is injured.