A peculiar girl transforms into a cat to catch her crush's attention. But before she realizes it, the line between human and animal starts to blur.

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.

14-year-old Arrietty and the rest of the Clock family live in peaceful anonymity as they make their own home from items "borrowed" from the house's human inhabitants. However, life changes for the Clocks when a human boy discovers Arrietty.