Ainz Ooal Gown, the undead sorcerer formerly known as Momonga, has accepted his place in this new world. Though it bears similarities to his beloved virtual reality game Yggdrasil, it still holds many mysteries which he intends to uncover, by utilizing his power as ruler of the Great Tomb of Nazarick. However, ever since the disastrous brainwashing of one of his subordinates, Ainz has become wary of the impending dangers of the Slane Theocracy, as well as the possible existence of other former Yggdrasil players. Meanwhile, Albedo, Demiurge and the rest of Ainz's loyal guardians set out to prepare for the next step in their campaign: Nazarick's first war… Overlord II picks up immediately after its prequel, continuing the story of Ainz Ooal Gown, his eclectic army of human-hating guardians, and the many hapless humans affected by the Overlord's arrival.

Mikoto Shinozaki is a high school student lacking in social awareness. In addition, due to his untidy appearance and low self-esteem, he is an easy target for bullies. Regardless, any misfortune that falls upon Mikoto is partly his own fault—at least, that is what Yuki Fukuzawa, the most popular girl in class, believes. With this assumption, Yuki invites Mikoto to join the school's bicycle club, in hopes he can improve himself and gain new friends. Motivated by feelings he has for Yuki, Mikoto has no reason to decline her offer—except for the fact that he does not know how to ride a bicycle. Despite his lack of even the fundamentals, Mikoto cannot get over the image of himself on a bicycle. Eventually, he succumbs to his curiosity and sneaks into the school grounds at night to practice. This begins Mikoto's journey as he pedals his way through the vast world of cycling, where the scenery of his life changes in ways he never thought possible.

It can be rough transferring to a new school—even more so if you don't want to make any friends, like Nobuaki Kanazawa. But the reason for his antisocial behavior soon becomes clear when his class receives a text from someone called The King. Included are instructions for the King's Game, and all class members must participate. Those who refuse to play, quit halfway, or don't follow an order in the allotted time of 24 hours will receive a deadly punishment. Having played the game before and watched as those around him died, Nobuaki tries to warn his clueless classmates. Unfortunately, they only believe him after the King's Game claims its first casualties. Stuck in a horrific situation with no chance of escape, Nobuaki has a choice: put his own survival above those around him, or do what he couldn't before and save his classmates.

The final hour of the popular virtual reality game Yggdrasil has come. However, Momonga, a powerful wizard and master of the dark guild Ainz Ooal Gown, decides to spend his last few moments in the game as the servers begin to shut down. To his surprise, despite the clock having struck midnight, Momonga is still fully conscious as his character and, moreover, the non-player characters appear to have developed personalities of their own! Confronted with this abnormal situation, Momonga commands his loyal servants to help him investigate and take control of this new world, with the hopes of figuring out what has caused this development and if there may be others in the same predicament.