Every 12 years, mercenaries who possess the highest caliber of brute strength, cunning wit, and deadly precision gather to participate in the Zodiac Tournament. Each warrior bears the name and attributes of one of the 12 animals of the Chinese zodiac. With their pride and lives on the line, they engage in vicious combat until only the victor remains. The 12th Zodiac Tournament begins in a desolate city, devoid of any evidence of the half million people who recently lived there. To raise the stakes, each warrior ingests a poisonous gem, thus setting a time limit on the tournament—and on their life. With one wish for the victor up for grabs, the Zodiac Warriors start their cutthroat battle for survival.

Despite completing all the tasks given to them, Ainz Ooal Gown, Tanya Degurechaff, Kazuma Satou, Subaru Natsuki, and the other members of Class 2 are surprised to find out that their role as students is far from over. With no means of returning home, the class of eccentric personalities is still mysteriously stuck in the unfamiliar world. Although, as they are becoming quite fond of each other, spending more time together does not sound that bad. With the unexpected arrival of new transfer students, the comedic antics of our beloved characters continue to grow. Thus, their bizarre yet nonchalant school life continues.

Former ace runner of Sendai Josei High School, Kakeru Kurahara is chased away from a convenience store for shoplifting. Shaking off his pursuer, he runs into Haiji Kiyose, another student from his university. Haiji is impressed by Kakeru's agility and persuades him to live in Chikusei-sou, the run-down apartment where Haiji resides along with eight other students. Having lost his entire apartment deposit at a mahjong parlor, Kakeru accepts the offer reluctantly. However, Haiji reveals a secret during Kakeru's welcoming party: the apartment is actually the dormitory of the Kansei University Track Club. He unveils his ultimate goal of participating in the Hakone Ekiden—one of the most prominent university marathon relay races in Japan. Unfortunately, all the residents apart from Haiji and Kakeru are complete running novices. Worse still, none of the inhabitants are even remotely interested in being involved with Haiji's ridiculous plan! With only months before the deadline, will the fourth-year student be able to convince them otherwise and realize his elusive dream of running in the Hakone Ekiden?

The dispassionate, transforming assassin Golden Darkness returns to peer deeper into the mysteries surrounding her new life, while a sinister Nemesis manipulates her younger sister Mea from the shadows. Along with their newly discovered mother, Tearju, this previously estranged family quickly becomes the center of everyone's attention. On the other hand, Princess Momo's Harem Plan stands on shaky ground amidst Rito's inability to confess to his longtime crush Haruna, who has grown feelings of her own. But things aren't as peaceful as they seem; an evil force looms amidst the innocuous commotion, threatening to eclipse the love, happiness, and friendship of Rito and his harem. Only the light of love can hope to banish the shadow.