When Keyaru acquired his powers as a Hero who specialized in healing all injuries regardless of severity, it seemed that he would walk the path to a great future. But what awaited him instead was great agony; he was subjected to years of seemingly endless hellish torture and abuse. Keyaru's healing skills allowed him to secretly collect the memories and abilities of those he treated, gradually making him stronger than anyone else. But by the time he reached his full potential, it was far too late—he had already lost everything. Determined to put his life back on track, Keyaru decided to unleash a powerful healing spell that rewound the entire world back to the time before he began to suffer his horrible fate. Equipped with the anguish of his past, he vows to redo everything in order to fulfill a new purpose—to exact revenge upon those who have wronged him.

As a child living in the town of Lawless, Angelo Lagusa has witnessed a tragedy: his parents and younger brother have been mercilessly slaughtered by the Vanetti mafia family. Losing everything he holds dear, he leaves both his name and hometown behind, adopting the new identity of Avilio Bruno. Seven years later, Avilio finally has his chance for revenge when he receives a mysterious letter prompting him to return to Lawless. Obliging, he soon encounters the Vanetti don's son, Nero, and seeks to befriend him using the skills he has quietly honed for years. Set during the Prohibition era, 91 Days tells the story of Avilio's dark, bloodstained path to vengeance, as he slowly ends each of the men involved in the killing of his family.

It is said that if someone seeks revenge, they should dig two graves—and this is especially true of those who use the Hell Correspondence website. At the stroke of midnight, this ominous web domain becomes accessible, and whoever inputs the name of their grudge will get a visit from Ai Enma, the Hell Girl. Ai explains that she will send the target's soul to hell, but the requester will also end up in hell when they die. For some, that price is far too steep; for others, where they will spend their afterlife has no bearing on their current, day-to-day hell. But one thing is certain: the Hell Girl will always be available to those in need of revenge, while she stoically performs her duty of ferrying souls into the darkness.

After moving into his new apartment, art school student Tatsuhiko Kido discovers a crack in his wall. When he peeks inside, he makes eye contact with his neighbor, Emiru Ikuno, as she masturbates. He rushes next door to clear the misunderstanding and inform her of the crack, but accidentally trips and falls over her in the process. To make matters even worse, Emiru now has photographic evidence that would plainly depict Kido as a sexual offender. Kido then learns that Emiru has a voyeurism fetish, and is blackmailed into leaving the crack as is. Unfortunately for Kido, it doesn't stop here! Emiru is also attending the same art school that he is, and he will be seeing her far more than he would like to. Emiru and Kido are set to embark on a unique friendship with wildly erotic potential.