A flashback shows a young All Might in Las Vegas, dealing with a couple of villains that robbed a casino, aided by his friend David who has worked with All Might and designed many of his suits.
Going back to the present, All Might brings Midoriya with him to a floating and mobile manmade city, called ‘I Island’, where they research quirks as well as hero supplemental items at the special ‘I Expo’ convention that is currently being held on the island. Upon arriving, All Might and Midoriya are greeted by David’s daughter Melissa, who welcomes the two and takes them to David.
After the reunion, All Might runs tests with David, during which he realizes that All Might’s Quirk is nearly depleted. He is horrified, believing that All Might will not be able to keep his role as the Symbol of Peace. Meanwhile, Melissa shows Midoriya around the island, and they run into Midoriya’s classmates from class 1-A, who were invited to the island for various reasons. After enjoying the island’s events, Melissa invites class 1-A to join a formal party being held with all heroes present. Before the party, Melissa gives Midoriya a gauntlet that allows him to use his Quirk at 100% without getting hurt.
Meanwhile, Wolfram, a 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 pro-heroes attending, including All Might, and takes David and his assistant Sam hostage in order to break into the island’s vault. Midoriya, Melissa, and the rest of class 1-A are able to avoid being restrained, and decide to fight the villains, and free the heroes, by reaching the top of the building and deactivating the security system. Wolfram deploys forces to stop them, but Midoriya and Melissa manage to climb to the top regardless.
At the top of the tower, they discover that David and Sam orchestrated the night’s events with actors. Their goal was to steal back their greatest invention – a headset that maximizes the power of a person’s Quirk, which had been locked away to prevent it from falling into the wrong hands. David wishes to give it to All Might, so that he could maintain his role as a hero. However, an arriving Wolfram reveals that he is not an actor, but a real villain working with Sam to retrieve the headset for himself. Wolfram steals the headset and attempts to kill Sam and Melissa, but David and Midoriya save them.
Wolfram holds off Midoriya, kidnaps David, and escapes to the rooftop. Midoriya fails to prevent Wolfram from taking off in a helicopter with David, but Melissa takes control of the security system and frees the pro heroes. All Might reaches the roof and stops the helicopter from taking off.
Wolfram then uses the headset to amplify his power, creating a giant metal body with David trapped inside. He overpowers All Might, revealing that he is secretly in league with All Might’s nemesis All For One, who gave Wolfram additional Quirks and orchestrated the night’s events to demoralize All Might. Lacking the strength to defeat Wolfram alone, All Might asks for Midoriya’s aid. The rest of the students soon arrive on the roof and help All Might and Midoriya, who combine their power to defeat Wolfram, freeing David.
As the sun rises, David and All Might reflect on how Melissa and Midoriya are the next generation of heroes and how the world will be in good hands even after All Might’s Quirk is gone.