You're my hero and shit

All Might and Deku accept an invitation to go abroad 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. During that time, suddenly, despite an iron wall of security surrounding the island, the system is breached by a villain, and the only ones able to stop him are the students of Class 1-A.

In a society devastated by the effects of an all-out war between heroes and villains, a mysterious giant fortress suddenly appears, engulfing towns and people one after another. Then, a man reminiscent of All Might, the 'symbol of peace', stands in front of Izuku and his friends...