Setsu was once summoned into the alternate world of Eclair, where he ended a war between humans, beastmen, and demons, and brought peace to the land. He was hailed as the hero who achieved world peace surpassing all racial barriers for the very first time, but after the war, he was sent back to his original world. Though he had to restart as a baby, he retained all his memories and powers from the other world.

Then, when he made it back to high school age, he was once again summoned to Eclair, where five years had passed and war threatened the land again...

[Source: Crunchyroll]