Shie Hassaikai — Overhaul

Students undertake dangerous internships with pro heroes, with Deku joining a mission against Kai Chisaki—the yakuza boss known as Overhaul. Chisaki possesses the horrifying Overhaul Quirk allowing him to disassemble and reassemble matter, which he uses to weaponize a young girl named Eri whose rewind ability erases Quirks. Deku must break new power barriers to save her and stop the imminent Quirk-erasing plague.

Kai Chisaki, known as Overhaul, leads the Shie Hassaikai yakuza faction and possesses an Overhaul Quirk capable of disassembling and reassembling matter at the molecular level. He applies this power to a young girl named Eri, whose Rewind Quirk allows her to rewind living things to previous states—potentially erasing Quirks entirely. Chisaki manufactures bullets infused with Eri's cells, creating weapons that permanently destroy Quirks. Quirk-erasing bullets represent an existential threat to hero society. If mass-produced, they could eliminate the Quirks of millions, destroying the foundation of the entire profession. The heroes must act decisively, but also protect Eri, a traumatized child who has suffered years of systematic experimentation and abuse. The mission becomes a rescue operation disguised as a criminal raid—saving a victim from a predator. During the fight against Overhaul, Deku experiences a breakthrough: his connection to Eri's Rewind Quirk allows him to use One For All at 100% power without destroying his body. For one transcendent moment, Izuku achieves what he's always dreamed of—full power without self-destruction. This moment is brief, devastating, and heartbreaking. Eri enables the victory through her own suffering and sacrifice. Mirio Togata, the strongest of UA's third-year students known as Lemillion, confronts the moment that defines his entire character. When an Overhaul operative attempts to hit Eri with a Quirk-erasing bullet, Mirio positions himself between them and takes the bullet instead. Mirio loses his Permeation Quirk—the power that made him heroic—choosing a powerless girl's life over his own future as a hero. This arc centers on sacrifice with no guarantee of reward or appreciation. Lemillion loses his Quirk and becomes a regular person in a society that measures worth by power. Deku briefly accesses overwhelming strength but cannot sustain it. Eri survives but remains traumatized. The victory is real but costs more than anyone wanted to pay. The arc asks: what is a hero willing to lose for someone else's safety? The answers vary, and those variations drive everything that follows.