The story takes place after the events of Season 2 of the anime, during the summer break. All Might, the number one Pro Hero and Symbol of Peace, receives an invitation to visit I-Island, a man-made island that serves as a hub for the world's top scientists and inventors who focus on support technology for Quirks. He brings along his protégé, Izuku Midoriya.
I-Island is hosting the I-Expo, a prestigious exhibition where scientists showcase their latest inventions. The island is heavily secured and designed to prevent the misuse of Quirk-related tech. All Might reconnects with his old friend David Shield, a renowned scientist who once worked with him during his early career in America. David is now working on new support gear and lives on the island with his daughter, Melissa.
Melissa Shield is a bright and energetic student with no Quirk, but she makes up for it with her engineering skills. She quickly becomes friends with Izuku. She gives him a support item that helps him better control his power output. As the expo begins, several other Class 1-A students arrive on the island, having received invitations through various channels.
During the gala event at the Expo, villains launch a surprise attack. They hack the island's security system, locking down the facility and taking many of the Pro Heroes hostage, including All Might. The villains are led by Wolfram, a mysterious man with the power to manipulate metal. With All Might restrained, it falls to Izuku, Melissa, and the other students to figure out how to stop the villains and restore the island's systems.
As the students split into groups to reach the central tower and deactivate the lockdown, they face off against the hired villain mercenaries. They overcome these obstacles using teamwork, their Quirks, and their wits. Melissa plays a key role in guiding them through the security systems and locating the override terminal.
In the final confrontation, it is revealed that Wolfram is not acting alone. He was secretly empowered by All for One, who gave him extra Quirks, making him much stronger than initially assumed. David Shield had secretly created a Quirk Amplification Device in the hope of restoring All Might's powers, fearing the era of peace would end with his retirement. He never intended it to be stolen or used for evil. However, Wolfram uses this device to push his own power even further, planning to steal other Quirks and dominate with brute force.
Izuku and All Might fight Wolfram in a climactic battle. Izuku and All Might unleash a double Detroit Smash to finally defeat him. All Might's secret about losing his power remains safe, but David Shield now understands the world still has hope—Izuku and the next generation of heroes are ready to carry the torch.