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?

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.

Set in an alternative world of Bungo Stray Dogs, this "what-if" story imagines if Atsushi Nakajima was in the Port Mafia and Ryūnosuke Akutagawa was in the Armed Detective Agency, instead of vice-versa. The story follows Ryūnosuke Akutagawa, who vows revenge on a man dressed in black in order to rescue his sister. However, as he is about to starve to death, a man from the Armed Detective Agency appears.