Plot
In the past, a young All Might, while as an exchange student in California, deals with a group of villains who have robbed a casino. He is aided by his classmate and friend, David Shield, who eventually becomes a scientist and works with him to design many of his suits.
In the present, during his summer break, All Might brings Izuku Midoriya with him to I-Island, a floating city island, in response to an invitation from David's daughter, Melissa, to surprise her father. During their reunion, David runs a test with All Might and learns that his Quirk is nearly depleted, becoming horrified as he believes that All Might will no longer be able to fulfill his role as the Symbol of Peace. Meanwhile, Melissa shows Izuku around the island and unexpectedly encounters Izuku's classmates from Class 1-A, who have all been invited to the island for various reasons. After enjoying the island's events, Melissa invites them to a formal party being held with all the Heroes present. Prior to the party, Melissa confesses to Izuku that she is Quirkless, a revelation that astonishes him due to him also originally being Quirkless. She then presents him with a "Full Gauntlet", a mechanical arm device that enables him to harness the full potential of his Quirk without suffering any harm.
Meanwhile, Wolfram, a terrorist villain who had secretly arrived on the island, takes control of the island's security system during the party and threatens to kill its residents. He restrains all of the attending Heroes, including All Might, and takes David and his assistant, Samuel Abraham, hostage to break into the island's vault. Izuku, Melissa, and some of Class 1-A at the party manage to evade the attackers, and they decide to save the hostages by reaching the top of the building and deactivating the security system. Discovering their presence, Wolfram deploys enemy forces to stop them, forcing the group to be split up as they deal with the Villains and security bots. Eventually, Izuku and Melissa are the only ones left who manage to climb to the top.
At the top of the tower, they discover that David and Samuel orchestrated the night's events. They had hired fake Villains to cause a distraction in order to retrieve their greatest invention – a headset that maximizes the power of a person's Quirk – which had been confiscated and sealed away by the project's sponsors. David wishes to give it to All Might so that he can maintain his role as the Symbol of Peace. However, Wolfram arrives and reveals that he is a real Villain, hired by Samuel to retrieve the headset for himself. He steals it and attempts to kill Samuel and Melissa, but David and Izuku save them. Wolfram holds off Izuku, kidnaps David, and escapes to the rooftop. Izuku fails to prevent Wolfram from taking off in a helicopter with David, but Melissa takes control of the security system and frees the Heroes. All Might reaches the roof and stops the helicopter from escaping.
Wolfram then uses the Quirk Amplification Device to amplify his power, creating a giant metal body with David trapped inside. He overpowers All Might and reveals that he'd also been given an additional Quirk by All Might's arch-nemesis, All For One, who wanted to be involved to demoralize All Might. With All Might lacking the strength to defeat Wolfram alone, Izuku comes in to aid him. The rest of the students soon arrive to assist in their fight, as All Might and Izuku combine their One For All's to defeat Wolfram and free David. As the students celebrate their victory, All Might and David reflect on the people, such as Melissa and Izuku, becoming the next generation of Heroes, and how the world will be in safer hands even after All Might's Quirk is gone.
