20 years ago.. there was a nuclear explosion that turned most people who had quirks into mutatans... citizens made groups to protect each other from other groups and mutants.

The ones who are the blame for the 'N-Boom' are the Mercidens. They're a large organization who kidnap survivors and kill them.

Katsuki is a part of the Hosu group, along with Denki, Eijirou, Todoroki, Izuku, Mina, Kyoka, and Iida.

The Hosu Group was exploring an abandoned city on foot...