Bungo Stray Dogs: The Untold Origins of the Detective Agency

Bungo Stray Dogs: The Untold Origins of the Detective Agency (文豪ストレイドッグス 探偵社設立秘話,) is the third light novel of the Bungo Stray Dogs series.

It tells the forming of the Agency 12 years before the main timeline, as well as the meeting of Yukichi Fukuzawa and Ranpo Edogawa. The novel also narrates Sakunosuke Oda's past as a young assassin.

Japanese
かつて横浜で用心棒をしていた福沢諭吉は、妙なことから傍若無人な天才少年・江戸川乱歩の面倒を見るはめに. 仕事で福沢たちが赴いた劇場で不可解な殺人が起き!? ２人の出会いと探偵社の日常を描く豪華２本立て！

English
Over a decade before the events of the manga series, a chance encounter leads professional bodyguard Yukichi Fukuzawa to begrudgingly take an arrogant boy genius named Ranpo Edogawa under his wing. Their first job brings them to a local theater on high alert after receiving a bizarre death threat — a challenge that will require both Ranpo's brains and Fukuzawa's experience. The aftermath spurs Fukuzawa to establish the Armed Detective Agency, an organization that takes on dangerous jobs even the police won't handle!

Plus, get a glimpse of the agency the night before Atsushi Nakajima joined the team!

Summary
At Café Uzumaki, Jun'ichirō Tanizaki asks Doppo Kunikida about the origins of the Armed Detective Agency, though Kunikida answers that he only knows that the President had an encounter some time ago that triggered the Agency's establishment. Later, Osamu Dazai arrives and tells them about a fake bomb incident earlier from a woman pining over him at a standing bar. Kunikida later asks what he plans on doing with Atsushi Nakajima, the homeless boy he took in. Dazai seeks out their help on what kind of entrance exam they shall have Atsushi take, after which they convene with the others in their office.
 * A Day at the Detective Agency

Tanizaki suggests having him solve a case among their requests, while Dazai pulls out an exam outline, both of which Kunikida chides for their impracticality. Kenji Miyazawa arrives and suggests testing the newcomer against his strength and other unfeasible tasks, which they all reject. Yosano recommends rather painful suggestions, while Kunikida proposes ideas having to do with assaulting Dazai, which they also reject. Naomi Tanizaki offers them some refreshments, so Dazai asks for her suggestions for the exam. She blushes upon suggesting three ideas, which are left undisclosed to readers.

In the end, they all agree to create an original exam themselves. Kunikida proposes that they draw lots, where the person who draws the lowest number pair gets the biggest role. In truth, fed up with Dazai's peskiness towards him for the past two years, Kunikida has planned with the Tanizaki siblings to make Dazai get the biggest role, cheating with an envelope prepared by Naomi. While drawing, Dazai suggests the biggest role to be a bomber using the fake bomb. He then gets the lowest among them; before Tanizaki can reveal his lot, Dazai asks Naomi to act as the bomber's hostage. In the end, Tanizaki sees that his lot is lower than Dazai, upon which Naomi is revealed to be behind the trick so that she gets to be her brother's hostage.

Later, Tanizaki finds a drawing at the reception desk done by Ranpo Edogawa about the optimal places where a bomber might barricade himself, which seems to be made before they even drew lots. Apparently setting his schedule to be away from Yokohama during the exam after seemingly seeing through it all, Tanizaki wonders how Ranpo came to believe that he has an ability when he actually does not. At night, Kunikida treats Dazai and Tanizaki to a pub, where they talk about the origins of the Agency again. A waitress serves Dazai his dish, where a note – similar to the fake bomb's note he received earlier – and a contraption with a motion-sensor cable is placed instead of food. He turns to his companions for help, only to find Kunikida and Tanizaki already gone.

Twelve years ago, Yukichi Fukuzawa is a lone wolf bodyguard renowned for his skill in martial arts. After his client, a company CEO, has been killed in her office, he heads to the crime scene, where he meets her secretary and the suspect tied up next door. An adolescent soon arrives to apply for a job, who later claims that the secretary is the real killer after stating his deduction. During a later skirmish with the suspect – a young assassin named Oda – the latter shoots the secretary to death. Fukuzawa subdues Oda and asks the adolescent to call an ambulance, after which the prodigy introduces himself as Ranpo Edogawa.
 * The Untold Origins of the Detective Agency

Fukuzawa treats Ranpo to red bean porridge, where the boy talks about his bewilderment towards adults, his expulsions at school and work, and his genius parents. Figuring that Ranpo is quite special, Fukuzawa allows the boy to come with him amidst his initial reluctance. They soon arrive at a theater, where Fukuzawa is hired to apprehend a criminal after receiving a death threat signed by an ambiguous "V". They are met by the manager, Ms. Egawa, who shows Fukuzawa the note regarding the murder, which Ranpo thinks will happen.

To gather information, Fukuzawa and Ranpo question the performers, starting with lead actor Tokio Murakami. During their inquiry, the youth states his willingness to risk his or anyone's life for the sake of mastering the art if given the chance. They also question the other performers, who think that no murder shall happen. However, Ranpo believes that the note is an announcement rather than a threat for murder, though he thinks that Fukuzawa, as an adult, must have already perceived that. To further assess his skills, Fukuzawa asks Ranpo to deduce him. As the boy uncovers his secretive past with ease, Fukuzawa fully believes that Ranpo is special.

The play begins, and Fukuzawa scans the theater to find the possible killer. He spots a suspicious man, but Ranpo stubbornly complains during the play. Though he tries to calm him, Fukuzawa soon finds Ranpo desperately shouting among the crowd on why nobody understands him, after which Fukuzawa brings him out of the theater hall. He discovers that Ranpo's parents essentially sealed away his skill for him to learn about the world like an ordinary person, knowing that society will come off as strange to him. However, Ranpo is left unguided after their deaths, causing him to misunderstand the world before he can fully comprehend it.

To rein him in, Fukuzawa convinces that Ranpo is special because he has an ability, which is to see the truth in one glance. He also gives Ranpo a random pair of glasses that he claims will help control his powers. Ranpo is later convinced that he, the world's greatest detective, must protect the simple-minded, ordinary people. He then asks Fukuzawa to return, while he sets out by himself to try and prevent the murder from happening. Fukuzawa returns, more vigilant than ever, as Ranpo assures him that the murder will occur in the next scene.

During Murakami's act, a knife suddenly pierces his chest, which is not part of the script. Fukuzawa rushes to him amidst the crowd's panic, who are later kept in place by the police as they seal the venue. The news emerges that Murakami has died en route to the hospital, while Fukuzawa worries that something might have happened to Ranpo, who has not yet returned. As he looks for him, he is informed that one of the audience members has escaped, who turns out to be the suspicious man he has spotted earlier. Fukuzawa thinks that the man might have taken Ranpo, but Egawa approaches him to relay Ranpo's instructions.

According to Ranpo, there are two criminals: one shabby and one terrible. Just like how a whale is caught with a shrimp, Ranpo plans on using the "shrimp criminal" to catch the "whale criminal". As Fukuzawa returns to his seat with several spectators, Ranpo appears onstage to explain the mystery. He reveals the alleged suspect (the man in the suit) to be a victim in the act and calls out the real perpetrator to appear, to which Murakami responds. Having faked his death with the playwright's help, Murakami confesses that he merely wished to portray the ultimate act: to perform a genuine death.

In the end, Murakami is taken away, and a police officer named Jun Mitamura thanks the two for their help. Ranpo replies that it is all thanks to his ability, but Fukuzawa tries to convince him that he is not an actual ability user, albeit to no avail. Mitamura takes Ranpo with him, while Fukuzawa learns from Murakami that the latter and the playwright were told by a third party about the man in the suit, after which Fukuzawa is told that the playwright has been found dead. Ranpo showcases his deduction ability to Mitamura; he deduces that the play was a mere trap to catch the man in the suit and that the playwright has been killed by Mitamura himself.

As Fukuzawa sets out to find Ranpo, he notices a note from the boy stating that Mitamura is the culprit and asks him to "look for the cane". Mitamura brings Ranpo to a deserted building, while Fukuzawa finds the cane of the man in the suit, where he finds a hidden computer memory chip. Mitamura invites Ranpo to work for them, but the boy declines. Fukuzawa visits Oda in his cell and inquires about his past clients who might have hired him to catch an ability user. Initially hesitant, Oda soon cooperates upon hearing that the mastermind is V.

Ranpo provokes Mitamura, after which the latter mentions how their gifted organization V aims to drive out fellow ability users who disrupt the foundation of the country. As Mitamura holds Ranpo at gunpoint, Fukuzawa arrives at the building and overwhelms Mitamura alongside five armed guards. Though Ranpo is thrilled with the bodyguard's prowess, Fukuzawa berates and lectures him never to risk himself to such danger again. In the end, the two make amends with each other.

In the aftermath, the case is closed thanks to Ranpo, though they soon learn of Mitamura's death in his cell, most likely by an ability user. Ranpo has also become attached to Fukuzawa from then on. Over the course of a year, Fukuzawa has brought Ranpo to several odd jobs, albeit later losing his job as a bodyguard. Nonetheless, various clients have sought Ranpo's detective skills alongside Fukuzawa's protection, eventually garnering the two considerable notoriety to criminal groups and individuals.

Later, Fukuzawa brings Ranpo to an underground hall, where they are greeted by the man in the suit. Ranpo is shocked with the man, while Fukuzawa seeks out the latter – Sōseki Natsume – for his help in acquiring a skilled business permit, as Fukuzawa has realized his desire to continue helping other people even if it required him to lead an organization. With Ranpo at the forefront, he aims to have him guarded and armed by gifted detectives suitable to carry out his vision, with which Fukuzawa establishes the Armed Detective Agency.

Post-Conflict

 * Sakunosuke Oda later quits his assassin job after meeting Sōseki Natsume.
 * Sometime after the Agency's establishment, Kunikida and Yosano join in.