A man named Dan was once celebrated as the strongest hero, but after years of living a lazy and self-indulgent life, he finds himself in a dire situation as a divorced, unemployed middle-aged man. To make ends meet, he decides to return to work as an adventurer. However, he quickly discovers that his old-fashioned fighting style and his age have left him out of touch with the modern era, to the point where he cannot even find a party to join.

His fortunes change when he is approached by Amber, a young and talented silver-haired elf who is also an S-rank adventurer. Amber recruits the down-and-out former hero, forming an unlikely partnership that marks Dan's return to the adventuring life. Their party soon expands to include Zenobia, the daughter of the very Demon Lord that Dan had defeated in the past. Zenobia is eager to help Dan regain his former status as a hero.

The story follows Dan as he navigates a world that has moved on without him, struggling to reconcile his legendary past with his current reality. The party takes on increasingly dangerous quests. One of their early major challenges involves a joint quest between the four great guilds, which is secretly a proxy war between dukes. The quest takes place in a forbidden region known as the Demon Lord's Treasury, where something even the Demon Lord could not handle is sealed away.

During this quest, the party encounters Via, one of the sealed strongest demons. Although Via initially appears friendly, her warped and dangerous nature soon becomes apparent. As the victors of the joint quest, Dan and his party are chosen as representatives of the nation and are sent to the barren, snow-covered land of Forbia. There, they meet one of Dan's former comrades, Grun, who tasks them with investigating an unprecedented abnormal situation plaguing the region.

The investigation reveals that the attributes of monsters are changing, leading the party into an icy labyrinth to find the cause. This quest unveils a larger conspiracy involving a new Demon Lord and the Eight Abyss, a group of powerful demons. Dan and his party must now confront this emerging threat, seeking information from old acquaintances among the Eight Abyss while some members, such as the yandere Lido who still loves the original Demon Lord and holds a deep grudge against Dan, prove to be hostile.