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.

It’s been roughly one year since Chuuya Nakahara joined the Port Mafia, and he’s determined to become an executive before fellow mafioso Osamu Dazai. However, a man by the name of Paul Verlaine—the king of assassins who claims to be Chuuya’s older brother—suddenly appears, vowing to murder everyone he cares about! Now Chuuya must team up with Adam, an AI investigator from Europe, to stop this madman before another storm engulfs Yokohama. Along the way, many truths will be revealed…including the answer to Chuuya’s most pressing question: Is he himself even human?

Not long after Ougai Mori takes the reins of the Port Mafia, his right-hand man, Osamu Dazai, has a nasty run-in with a gravity manipulator named Chuuya Nakahara, leader of the Sheep gang. However, with ominous rumors about Mori's deceased predecessor as well as the mysterious being called Arahabaki spreading throughout Yokohama, the duo must put aside their differences and get to the bottom of things. Before they achieved infamy as the fearsome pair Twin Dark, Dazai and Chuuya were just boys. Was their first encounter a harbinger of hope or an ill omen of things to come...?

Takatora Kureshima (Kamen Rider Zangetsu) visits a former Yggdrasill Corporation experimental project site located in the poverty and conflict stricken Republic of Torukia. He is visiting the site for the first time in 8 years, and is suddenly attacked! Suprised he stumble and falls into a giant hole which leads into an underground city. This underground city is the most dangerous place in the republic, in which many young men are killed defending themselves while trying to survive. These men use Lock Seeds and Drivers to transform into Armored Riders. Due to the fall Takatora has lost his memory. He is helped by the leader of Team Orange Ride. Unbeknownst to them, there are cameras hidden all around the underground city. The ones behind the cameras are responsible for the conflicts in the city. One man in particular is seeking revenge on Takatora for a specific reason...

A criminal organisation called the Rats in the House of the Dead makes its existence known to the Armed Detective Agency. Little is known about it and its alleged leader, Fyodor Dostoevsky, but a downward spiral begins when the leaders of the Agency and the Port Mafia are infected with a virus named Cannibalism and are left gravely ill. With just 48 hours to save their leaders, a blood feud breaks out between the organisations of Yokohama.

After the events of the series, Shoma continues his research for "Light Treats" to replace Dark Treats, Hanto continues his research as a freelance writer as keep daily records of the battle of the Kamen Riders, and Rakia remains alone in the Granute World. However, a new incident occurs in the once peaceful human world, forcing Shoma to rush back to the battlefield. What serious abnormality has occured to Hanto's body? What is the current state of the president's daughter, Lizel Jaldak, who is stuck in the human world? The final battle of the candy Kamen Riders begin!

Based on the in-game event Special Investigation: Keiou Koufu.