The League of Villains are pursued by several Heroes while driving a truck carrying a life support capsule. Endeavor arrives and manages to destroy the truck, with the Villains revealed to be clones created by Twice as it crashes. The figure within the capsule, a villain named Nine who allowed himself to be experimented on by Daruma Ujiko, escapes during the crash and regroups with his team of villains to fulfill their dream to create a society ruled by those with strong Quirks. While Ujiko gave him a copy of All For One's Quirk, capable of acquiring eight more Quirks besides his own Weather Manipulation, Nine needs a special Cell Activation Quirk to cure himself of a terminal disease that worsened as a side-effect of his modification.
U.A. High's Class 1-A has been sent to the remote Nabu Island as part of a safety work program. Izuku Midoriya, the current wielder of One for All, meets Mahoro Shimano and her younger brother Katsuma, residents of the island. Despite some hostility from the sister, Izuku bonds with them along with his rival Katsuki Bakugo, and discover that Katsuma wishes to become a hero, although Mahoro seeks to dissuade him, fearing for his safety. Meanwhile, Mahoro and Katsuma's father is attacked by Nine's group and his Quirk is taken, but the Quirk is incompatible with Nine's blood type. Learning about the man's children on Nabu, they theorize that one of them would have the Quirk with the right blood type.
Nine's gang arrives on the island and destroy all means of escape and communication. Class 1-A learns of the invasion and split up to stop the villains and protect the island's residents. Nine finds the children and confirms Katsuma possesses the Quirk he seeks, but Izuku intervenes. Nine ends up greatly overpowering Izuku with his multitude of Quirks, even with Bakugo's help, and they are defeated. However, Nine is forced to retreat after overusing his Quirks, allowing Class 1-A to save Izuku, Bakugo, and the Shimanos. After 1-A regroups, they deduce why Nine wants Katsuma after he uses his Quirk to heal Izuku and Bakugo. They decide to prepare and fight the villains while awaiting the arrival of other Heroes.
After evacuating the islanders to a small island, the class successfully separate Nine from his gang, with them each being defeated by the students. However, Nine is successfully able to incapacitate all the remaining students, and once again overpowers Izuku and Bakugo by ingesting Quirk-empowering drug fluids, threatening to destroy the island. Seeing no other way to defeat Nine, Izuku transfers One For All to Bakugo, while he uses its leftover embers. Together, Izuku and Bakugo finally defeat Nine, with Izuku's One For All seemingly fading afterward.
As reinforcements from the mainland arrive, All Might recovers an unconscious Bakugo and Izuku, and realizes that One For All remains within Izuku, as the transfer into Bakugo was interrupted, but theorizing that the previous wielders of One For All wish for Izuku to keep it. Elsewhere, Tomura Shigaraki finds and kills the weakened Nine out of spite, vowing to do what he failed to do. With Nine's gang apprehended, the class help repair the damage done to the island before returning home. Izuku and Bakugo, who had lost the memory of wielding One For All, say goodbye to Katsuma and Mahoro, as Izuku assures Katsuma he can become a hero, just like All Might had done for him previously.
Source: Wikipedia
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