Port Mafia Arc

The Port Mafia Arc (ポートマフィア編,) is the first story arc in Bungo Stray Dogs.

Synopsis
It showcases the protagonist, Atsushi Nakajima, as he adapts to live with his ability and meets both the Armed Detective Agency and Port Mafia, becoming the epicenter of their latest conflict.

Man-Eating Tiger at Yokohama
A young orphan is seen starving beside a river and decides to steal money from the next man who passes by. However, he sees a drowning man and saves him. It was then revealed that the drowning man is attempting to commit suicide but is saved by the orphan. As the drowning man wanted a clean suicide and the orphan was entangled to it, he offers to buy him a meal using his coworker's money who just arrived at the scene looking for him. The drowning man's name reveals to be Osamu Dazai and the orphan's name is Atsushi Nakajima.

Later at a soba restaurant, Dazai and his coworker Doppo Kunikida reveal to be detectives from the Armed Detective Agency, a detective company with ability users who is currently seeking a case about a man-eating tiger. Shocked by what he heard, he tries to run away but is restrained by Kunikida. Atsushi tells them that he knows the tiger as it is the one who cause wreckage to the orphanage where he came from and followed him until Yokohama. Shortly after, Dazai offers Atsuhi to join him in their tiger hunt as it has compensation.

At the warehouse, Dazai and Atsushi are seen waiting for the tiger to appear. As the moonlight appears, Atsushi panics as the tiger will appear soon. Dazai tells Atsushi that it is not a coincidence that the tiger appears just as where Atsushi is and reveals that Atsushi is an ability user who can transform into a tiger. Atsushi eventually transforms and attacks Dazai but Dazai nullifies it. Shortly after, the other agency members including Ranpo, Yosano, and Kenji arrive at the scene.

Atsushi's Entrance Exam
After being taken by the agency, Atsushi receives a call from Dazai and meets up with him. Dazai tells Atsushi that he is about to secure a work him. Just then, Kunikida finds them and immediately informs them about a bomber holed up in the agency.

Dazai, Kunikida, and Atsushi soon arrive at the agency. Kunikida tries to confront the bomber in attempt to save the hostage however the bomber knows Kunikida's identity. While Atsushi and Dazai are hiding from sight, Dazai tells that the bomber may have an issue with the agency and his appearance will only cause more harm. As a result, Atsushi is the one who approaches the bomber trying to stop him and making him surrender. Due to the distraction, Kunikida manages to activate his ability and restrain the bomber.

The timer on the bomb stopped but someone had pushed Atsushi causing the timer to start again. Panicking and not sure what to do, Atsushi covers the bomb with his body. After the bomb reaches to zero, it didn't explode. It is revealed to be Atsushi's entrance exam to the agency. Shortly after, the president appears and leaves the decision whether Atsushi passed the exam to Dazai. Dazai then welcomes Atsushi to the agency.

Port Mafia's Appearance
The other agency members introduce themselves. The disguised bomber reveals to be Jun'ichirō Tanizaki and the hostage is his sister, Naomi. Atsushi asks the members what their job is before joining the agency. The Tanizaki siblins are both students, Kunikida formerly taught mathematics at a licensed school. However, Atsushi can't guess Dazai's job to which Kunikida describes Dazai's prior vocation is one of the great mysteries of the agency.

Shortly after, the agency receives a client request to investigate something. The young woman states that there are people of low character hanging out behind their building. Kunikida gives the job to Atsushi, accompanied by Tanizaki whose sister also decides to tag along. Before the stakeout, Kunikida gives a picture of a young man to Atsushi and tells to never encounter the person., if he does, he must flee. Kunikida reveals the young man's name to be Akutagawa, one of Port Mafia's attack dogs.

Meanwhile at a police station, Akutagawa delivers a case and kills the policemen. After leaving, a woman enters the station and sees the policemen dead. Soon after, the police station explodes and Akutgawa is seen talking to someone over the phone asking what's the next order.

The client soon leads them to the company building however Tanizaki finds the place suspicious and asks if it is really the place. The young woman introduces herself as Higuchi and tels them that her real goal is to get all of them. She speaks to Akutagawa through phone telling that she have captured them.

Higuchi proceeds to shoot them. Naomi manages to save Tanizaki by shielding her body against him. As a result, Tanizaki panics and tells himself that he needs to get Naomi treated. Just then, Higuchi corners him pointing a gun to his head. While carrying Naomi, Tanizaki acitivates Light Snow. Unable to pinpoint Tanizaki form the snowfall, Higuchi is strangled by Tanizaki from behind. However, just then Akutagawa comes to Higuchi's aid and attacks Tanizaki.

Akutagawa introduces himself to Atsushi and reveals that their goal is to capture Atsushi alone.Akutagawa activates Rashōmon and attacks Atsushi. Atsushi and Akutagawa engage in a battle - with Atsushi turning himself into a tiger.

In the middle of their battle, Dazai appears and nullifies both of their abilities. It is revealed that Dazai had put a bug, an eavesdropping device to Higuchi and saw through her whole plan. Akutagawa declares that they will retreat for now but will return to capture Atsushi once more. Dazai asks their reason for capturing Atsushi and Akutagawa reveals that Atsushi has a reward of 7 billion yen on the black market. It is then revealed that Dazai is an ex-mafia.

Attack of Black Lizard
After the encounter with Akutagawa and realizing that he may be putting the agency in danger, Atsushi contacts Higuchi and tells that he will be leaving the agency. Higuchi orders to get the Black Lizard and gives them the mission to destroy the agency's office - insisting that they must be eliminated first since they failed on their first attempt. The Black Lizard arrives at the agency office and raids it.

Meanwhile, while Atsushi is leaving, he hears gunshots from the agency office. He soon rushes to the agency office only to find the Black Lizard and other mafia guards defeated by the agency members.

Yamagiwa Murder Case
While cleaning up the agency, Ranpo is headed to a case. Kunikida orders Atsushi to accompany Ranpo as he doesn't know how to use the train. The two soon arrive at the scene who are welcomed by Officer Sugimoto and Minoura. Minouara briefs them about the incident in which the victim is his subordinate.Soon after Minoura's claim about Ranpo describing himself as the the greatest detective without showing it, Ranpo activates his ability and reveals the culprit to be Officer Sugimoto. He further states his claim about how Officer Sugimoto would explain his three missing bullets.Instead of showing his handgun, he points it to Ranpo to which Atsushi is able to restrain him quickly.

Officer Sugimoto is then brought to police station for interrogation. It reveals that he tried to stop Yamagiwa from investigating a corrupt case in which Sugimoto is a spy for. As a result of trying to persuade Yamagiwa to drop the case, Sugimoto accidentally shot Yamagiwa when the latter stop Sugimoto from shooting himself in order to settle the issue.

Port Mafia's Bomb Threat Scheme
Atsushi accompanies Yosano during her shopping. On their way back to the agency, they hear an announcement on the train about an experiment. Suddenly, part of the train explodes causing many on-board passengers to die. The speaker addresses Atsushi and gives him the option to present himself otherwise all passengers will be killed. Yosano then instructs Atsushi for them to split and defuse the bombs.

On the way to the front rear of the train, Yosano is attacked by lemon bombs, who reveals to be Motojirō Kajii's doings, a Port Mafia member. Meanwhile, Atsushi encounters a young girl and attacks Atsushi.

Atsushi is continued to be attacked by the young girl whose name reveals to be Kyōka. Meanwhile, Kajii continues to attack Yosano with lemon bombs, eventually leaving her to die. When Kajii comes back, he finds Yosano alive and gets punched by her, eventually resulting to his defeat. Heavily injured, Atsushi sees all the passengers at the train and gets an idea that if he manage to save all of them, then it will be okay for him to keep on living.

Atsushi faces Kyōka and uses his ability to counter her Demon Snow's attacks. He eventually corners her and asks the location of the bomb. Kyōka reveals the bomb strapped to her chest. Atsushi orders Kyōka to give the emergency switch as relayed by Yosano about defusing the bomb. Atsushi presses the emergency switch however a voice from Kyōka's phone tells that they will not be defusing the bomb.

Seeing that the bomb does not stop, Kyōka jumps off the train and reveals her true feeling about not wanting to kill anyone ever again. Just then, Atsushi jumps off the train and gets the bomb off from her chest. Meanwhile, Dazai is captured by the mafia.

Kidnapping of Atsushi
After Kyōka is taken to the agency's custody, the agency learns that Akutagawa is responsible for controlling Kyōka's ability through phone. Kunikida then tasks Atsushi to turn her over to the military police. Meanwhile, Dazai is confronted by Akutagawa suring his capture. Atsuhi takes Kyōka on different places and they eventually arrive near the police station, just then, Atsushi is pierced by Akutagawa's Rashōmon eventually kidnapping Atsushi.

After learning that Atsushi has been taken, the agency investigates on the matter. With Fukuzawa persuasion to Ranpo, Ranpo uses his ability and is able to locate Atsushi. Apparently, Atsushi is kidnapped using one of Karma Transit's trucks and is currently in a ship heading to a foreign nation. Fukuzawa instructs Kunikida to get Atsushi back.

Meanwhile, Chūya Nakahara visits the detained Dazai and challenges him into a fight. He eventually asks Dazai the reason he lets himself get caught by the mafia. Dazai reveals that it has to do with Atsushi. After blackmailing Chūya,Chūya lets Dazai go and reveals that Akutagawa is the one governing regarding Atsushi's capture and records of it are in the archive storage room.

Atushi eventually wakes up after being abducted and sees himself at a ship. Akutagawa shows his anger and hatred towards Atsushi. Kyōka orders Akutagawa to let Atsushi go but was overpowerd and strangled by Akutagawa instead. Shortly after, Kunikida arrives to rescue Atsushi. While being strangled, Kyōka triggers the switch causing part of the ship to explode. Kunikida insists Atsushi to let Kyōka go as he can't save everyone. However, Atsushi decides to come back and rescue her just like how the agency didn't abandon him. Atsushi saves Kyōka from Akutagawa and challenges him to a fight.

Akutagawa and Atsushi engage in a fight. During the battle, Akutagawa continuously shows his hatred towards Atsushi. The battle eventually results to Akutagawa's defeat. Kyōka assists Atsushi to escape, eventually being rescued both by Kunikida.

Meanwhile at the Port Mafia base, Dazai goes to investigate regarding the bounty on Atsushi. It reveals that an American organization by the name of Guild is responsible for it, together with the participation of the Order of the Clock Tower and Rats in the House of the Dead.

Car Explosion Case
The agency receives a work request from Minoura. Kunikida offers Atsushi the job together with Kenji. The two arrive at the scene and finds a car blown all the way up of a building. After checking the scene, the two starts to gather some information about the incident. During their investigation, they are approached by Kenji's friend. Kenji proceeds to ask him if he knows anything about the car explosion. Kenji's friend reveals that he saw some guys buying loads of fertilizer to which Kenji tells that someone can make bombs with those.

Kenji and Atsushi proceed to investigate a spot of the district where a young men's association gather. After arriving at the spot, Kenji asks the gang members if they know anything about the car explosion to which all of them denies the accusations. After hearing their statement, Kenji and Atsushi leave. On the way back to the agency, they are cornered by the gang members who reveals to be the culprit. Kenji give them thanks for telling him the truth. Eventually, Kenji defeats all the gang member, much to Atsushi's suprise.

Karma Transit's Attack on Akutagawa
After Akutagawa's defeat from Atsushi, he is rescued by Higuhi. Akutagawa became comatose and received multiple fractures and burns across his whole body. After the failed bounty mission, Higuchi is interrogated by the Port Mafia boss.

Soon after, Higuchi learns that Karma Transit is planning to attack Akutagawa by hiring a foreign mercenary group. Due to this, Higuchi still decides to fight them on even though was stopped by the Black Lizard.

She later arrives at their hideout and fights the group alone. Black Lizard soon appears on her aid killing all the mercenaries. They successfully avoid Akutagawa being killed by the group. Akutagawa soon wakes up and apologizes to Higuchi.

Impact

 * Atsushi Nakajima discovers his ability and joins the Agency.
 * The Armed Detective Agency is introduced.
 * Port Mafia is introduced.
 * Dazai's past occupation as a member of the Port Mafia is revealed.
 * Atsushi Nakajima and Ryunosuke Akutagawa develop their rivalry.
 * The rivalry between Osamu Dazai and Chūya Nakahara is introduced.
 * Kyōka Izumi defects from Port Mafia.