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.

Fleeing from the consequences of his decision at the Hell's Gate, superpowered Contractor Hei and his companion Yin take refuge in a quiet inn, adopting the guise of a married couple in order to not draw suspicion. In an attempt to recover from recent events, Hei befriends the inn's other guests. He discovers that one of them is a fellow Contractor tasked with killing him. Their resulting encounter spells disaster for both Hei and Yin, who are forced to fight for their lives and grapple with the emotional wounds sustained in their previous life together.

In his past life, Prince Lloyd de Saloum was a commoner who could not become adept at magic, no matter how knowledgeable or obsessed he was about it. Now reincarnated into his current royal lineage, he receives a body with seemingly endless mana, making his desire to master all things arcane attainable. Moreover, as the seventh prince of the kingdom, Lloyd has no claim to the throne, allowing him to nurture his abilities as freely as he wants. Unfortunately for Lloyd, ancient seals that imprison powerful demons begin to break down and release the horrors within, endangering the peace. With these monstrosities roaming around, Lloyd's overwhelming magical prowess is the ultimate weapon that can neutralize these threats before all things descend to chaos.

Having attended Mihama Academy for about a year, Yuuji Kazami has seemingly found his place within the school, but he suddenly decides to pursue a promotion in CIRS. After consulting JB about his intentions, they both thoroughly examine Yuuji's documents and dissect the events of his upbringing to determine if the job is fit for him. Meanwhile, unbeknownst to the two, the girls of Mihama uncover some torn documents in Yuuji's room. After restoring the papers, they discover the story that has formed—or perhaps broken—Yuuji into the man he is today. However, what was thought to be history has haunted him to the present, and the chains of the past begin to drag him back into the darkness...