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.

Two men have just arrived at a location known as Quindecim and are unable to remember how they got there. They are immediately greeted by a young woman who escorts them to a small bar, where a bartender awaits them. They are told that they will have to participate in a game, randomly chosen by roulette, and will be unable to leave until its completion; if they refuse, the consequences will be dire. In addition to the rules of the game, the two men are told to play as if their lives are at stake. The game that has been chosen is billiards. But there's more to it than just pocketing pool balls, as the two are about to find out the outcome could mean life or death.

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.

Tokaku Azuma has just transferred to the elite Myoujou Academy, a private girls' boarding school. But there's a catch: she, along with 11 of her fellow students in Class Black, is an assassin taking part in the challenge to kill their sweet-natured classmate, Haru Ichinose. Whoever succeeds will be granted their deepest desire, no matter the difficulty or cost. However, each assassin only gets one chance; if they fail to kill her, they will be expelled. Despite the extraordinary reward, Tokaku decides to take a different course of action. Though Haru is her target, the young assassin soon finds herself drawn to the very girl she is supposed to kill. With the entire class out for Haru, Tokaku refuses to let her friend die, vowing to protect her from a growing bloodlust.