Famous writers throughout history find themselves being reincarnated by a mysterious, unseen entity known as the Alchemist. With their souls confined and bound to an expansive library, they are tasked by the Alchemist to jump into books to purify the pages of monsters called Taints. Along the way, they must also rescue and recruit fellow authors trapped within the very stories they themselves had written. Although the writers take on new and powerful forms for this endeavor, some still maintain a semblance of who they once were, while others struggle to remember their pasts and the works they had penned. Despite there being no apparent end to their grand mission, they remain committed to the cause in hope of resolving the mystery behind their collective resurrection as well as questions that have haunted their former lives.

A large-scale catastrophe is occurring across the planet. Ability users are discovered after the appearance of a mysterious fog, apparently having committed suicide, so the Armed Detective Agency sets out to investigate these mysterious deaths. The case seems to involve an unknown ability user referred to as Collector, a man who could be the mastermind behind the incident. Trust and courage are put to the test in order to save the city of Yokohama and ability users across the world from the grip of Collector where the Armed Detective Agency forms an unlikely partnership with the dangerous Port Mafia.

Although citizens of London view dragons as a fairy-tale myth, statistics confirm that 72% of the city's deaths are caused by these grotesque beings. Unable to see them, the public is oblivious to their existence. However, in a mirror dimension to Front London, exists a place where dragons can be seen with the naked eye—Reverse London. Ninny Spangcole is a member of a popular girl group. But in Reverse London, she works as a Witch at Wing Bind—an organization that dispatches agents to exterminate the beasts and protect the citizens of both Londons using magic. Ninny and her partner, Noel Niihashi, in addition to their jobs, safeguard Baldo Ywain Parks, a young man with an odd connection to the dragons. Thanks to the Wing Bind's hard work, there were no fatal dragon attacks for almost a century. But the peace shatters when Baldo's presence unexpectedly causes a Dark Dragon to wreak havoc in the city. The witches are further inconvenienced when Ninny's troublesome former bandmate appears in Reverse London—in tandem with another powerful dragon.

Armed Detective Agency members discuss the most suitable candidate for the second-in-command. Doppo Kunikida is carrying out official errands as planned in his diary as usual. Unexpectedly, a bomb-related incident occurs, challenging the ideals he has always upheld. When weighing one life over hundreds, how will he proceed?