The Unforeseen Simulation Joint invasion serves as the brutal awakening for Class 1-A, but for Hisoka, it is a catastrophic failure of the system he relies on. When the portal villain scatters the students, Hisoka uses his sand network to cushion their falls, but he cannot save Koji Koda and Rikido Sato, who are killed by the villains. The reversal hits hard when Aizawa, battered and broken, tells the surviving students to quit, only for the class to stand united in defiance. Hisoka stands with them, but his internal monologue reveals a crushing guilt; he feels responsible for their deaths because his mind was entirely focused on his own hidden agenda rather than protecting his peers. During the attack, Hisoka threads his sand through the facility, maintaining multiple perspectives to track the scattered students. He watches Eraserhead sacrifice himself to buy them time, a stark contrast to Hisoka's own selfish motivations. The death of Koda and Sato leaves an empty desk in the classroom that haunts Hisoka, serving as a constant physical reminder of his inadequacy. This emotional gut-punch destroys his belief that he can safely navigate the hero system while keeping his secrets. The realization that the legal framework of heroics is too slow and fragile to stop organized violence pushes him to abandon his cautious approach. He decides he must acquire power and information by any means necessary, setting up his descent into vigilantism during his upcoming internships.

Hisoka deliberately tailors his Sports Festival performance to attract the Hawks Agency in Fukuoka, using the internship as cover to stalk Kimiko Habiki, the mother of a suspected trafficker. Unable to gather evidence legally, he corners Inaba, a teenage thief with a memory-reading quirk, and blackmails her into infiltrating Kimiko's home to read her mind while she sleeps. Hisoka uses his sand network to map the entire city, tracking Inaba's movements and discovering the squalid hideout where she and her brother are kept by a ruthless gang. He confronts her in an alleyway, using his omnidirectional vision to recite the exact PIN codes and addresses she stole that night, proving he has absolute leverage over her. The reversal here is Hisoka's internal shift when Inaba breaks down in absolute terror, expecting him to hurt her. Looking at her crying, Hisoka realizes he has become the very monster he is hunting. He consciously channels the empathy of his classmates, specifically Eijiro Kirishima, to comfort her and promise to rescue her younger brother. He crosses the moral event horizon by committing breaking and entering, blackmail, and kidnapping, but he anchors his humanity by successfully extracting Inaba and her brother. The hook left behind is the information Inaba steals from Kimiko's mind: a letter declaring the death of Kaito Habiki, signed by a mysterious boss, which points Hisoka directly toward I-Island and a scientist named Susumu Hoshi.

Hisoka travels to I-Island under the guise of a school trip, intending to kidnap and interrogate scientist Susumu Hoshi. His meticulous plan is derailed when David Shields stages a fake terrorist attack to steal a Quirk Upgrader, taking the entire island hostage. The massive reversal hits when Hisoka infiltrates the tower and overhears David explaining that the unique base for the Upgrader serum is blood donated by an anonymous philanthropist. Hisoka instantly deduces that the blood belongs to Nanami. David Shields, trying to save All Might's fading power, unknowingly used the blood of the girl Hisoka has been hunting for eight years. The confrontation in the vault is chaotic, with Wolfram tearing the room apart with metal manipulation while the U.A. students fight for their lives. Hisoka ignores the battle entirely, focusing solely on securing the briefcase containing the serum and abducting David. He drags the scientist out of the shattered window, using his sand to control their descent to the rooftops below. The emotional peak is a blinding rage that causes Hisoka to force David to lead him to Susumu's hidden lab. The gut-punch lands when Hisoka finally breaches the basement and finds Nanami alive, missing an arm but still possessing her radiant spirit after eight years of captivity. The consequence of his reckless, unauthorized rescue is his immediate arrest by island security, leaving him under house arrest and facing expulsion from U.A. High School.

Confined to his manor, Hisoka finally confesses his years of lies, illegal investigations, and vigilante crimes to Uncle Sajin, expecting rejection but receiving profound forgiveness. This emotional resolution clears his conscience just as All Might and the Hero Public Safety Commission President arrive to recruit him. They need him to interrogate Susumu Hoshi in Tartarus about All For One, the legendary villain who steals quirks. The interrogation room in Tartarus is a sterile, heavily monitored cage, but Hisoka turns it into his own theater. He uses his quirk to sculpt a perfect, moving sand replica of Nanami as a child, forcing Susumu to confront the humanity of the girl she kept in a basement for nearly a decade. Susumu's rapid eye movements and clinical detachment crack under the weight of Hisoka's silent, omnidirectional pressure. Through this psychological dominance, Hisoka confirms the link between Susumu's trafficking ring and All For One's quirk-hoarding empire. He realizes that Nanami's quirk was kept inside her body because its amplification potency grew with age, making her a living battery for the Upgrader serum. This revelation shifts the stakes from a personal rescue mission to a national security crisis. The story leaves off with Hisoka stepping fully into the shadow war, no longer a rogue vigilante, but a sanctioned weapon armed with the truth, the backing of the highest authorities, and the unwavering support of the friend he finally brought home.