The now 18-year-old Saki Asamiya, formerly known as Yoko Godai, has stepped away from her high-octane life as a Sukeban Deka heroine to focus on college entrance exams and try to return to a normal existence. Yet danger finds her again when she crosses paths with Kazuo Hagiwara, a young man fleeing a group of hitmen from Sankou Academy, a private school perched on a remote island whose ominous reputation has earned it the nickname Hell’s Castle. The island’s control rests with the ruthless headmaster Hattori, a terrorist figure who aims to brainwash students into his loyal agents, bending the school into a dangerous machine for his political ambitions. The chance encounter pulls Saki into a web of conspiracy that stretches beyond a single rescue.
Their escape from Sankou’s henchmen aboard a bus becomes a brutal test of resilience, as Saki and Kazuo endure electric torture but manage to break free, sparking a renewed resolve to thwart the sinister plans unfolding on the island. Realizing the danger he’s in, Saki calls in trusted allies from her past: her longtime friend Kyôko Nakamura, known as Marble Okyo, and they soon reach out to Megumi Katô, the sister of Kazuo’s friend Kikuo Katô, who has his own history with Sankou. The situation intensifies when they’re joined by Yui Kazama, Yoko’s successor as Saki Asamiya, who has been sent by the enigmatic Dark Inspector. The team’s fragile sense of unity is tested as they navigate a maze of loyalties and betrayals on a remote island.
Before they can act, a helicopter dispatched by Hattori strikes, and Kazuo sacrifices himself to shield Saki from the onslaught, a moment that deepens the emotional stakes of their mission. Reunited with Yukino Yajima, the group hatches a plan to raid Sankou Gakuen and bring down Hattori once and for all. Nishiwaki reveals a chilling truth: Hattori is not merely a tyrant but the resurrected revolutionary leader Jishu Kita, now backed by a shadowy political fixer known as the Mastermind. He also delivers an upgraded weapon to Saki—a quadruple-weighted yo-yo with 16 times the power of her previous tool, a gift that promises great capability but carries the risk of irreparable damage to her arm.
The infiltration sequence unfolds with grit and peril as the team fights their way onto the island. A shocking betrayal by Megumi—who turns on the group to reconnect with her brother Kikuo—tears at their plans, yet Megumi’s loyalties swing back when she discovers the true fate of Kikuo, who has been lobotomized. The battle intensifies as the squad assaults Sankou Gakuen and its cadre of elite juvenile fighters. Megumi makes the ultimate sacrifice to save Saki from a lethal dart launched by Hattori, a moment that reshapes the course of the confrontation and fuels Saki’s determination to end the threat.
In a climactic showdown, Asamiya confronts Hattori once more, wielding the enhanced yo-yo and delivering a decisive electric strike that defeats him, even as she nearly cripples her own arm in the process. With Sankou Academy destroyed and the Mastermind suicidally choosing death to avoid capture, the immediate danger retreats. The island’s grip loosens, the students are freed, and the fallout signals a turning point for all involved. At journey’s end, Yoko parts ways with Nishiwaki—who jokes that this really is goodbye—while stepping forward with Kyoko and Yukino, each walking away into an uncertain but hopeful future, their fates now intertwined by the battles they endured together.
