20 years ago, the final battle between good and evil was held. All For One, the demon king of the underworld himself, and his protégé Tomura Shigaraki, against the combined forces of the world's heroes. Many sacrifices were made that day, but in the end they were not in vain as All For One was defeated, and Tomura Shigaraki fled into the shadows.
Some years later, Izuku Midoriya stands proud as he is announced the number one hero in Japan, solidifying his legacy as the next All Might. Thus, there has been a time of peace since that day.
20 years since that final battle, many of the young heroes from then have had their own children. The next generation of heroes ready to seize the day and build up their own legacies. But all the while dark forces have made moves in the shadows, and are preparing to reveal themselves to the world once more.
And so, welcome, to the new age of Heroism.