Osamu Dazai vs. Big Man

Osamu Dazai vs. Big Man is a battle fought between Armed Detective Agency rookie Osamu Dazai and an unnamed big man who is part of an underground criminal organization affiliated in the organ-trafficking syndicate.

It happened at an isolated military base during the Agency's investigation on the Azure Apostle messages and, concurrently, as part of Dazai's entrance exam.

Prologue
After receiving a message from the Azure Apostle that requests the Agency to prevent passenger airline flight JA815S from crashing into Yokohama, Dazai and Agency member Doppo Kunikida track the transmitter signal paralyzing the aircraft to a former National Defense Force military base. Upon arriving, their vehicle is immediately attacked by armed men. Kunikida then materializes and tosses a flash grenade from his notebook at them, giving Dazai a chance to head to the building's maintenance depot inside.

Following the tracking device that leads him to the base's operations room, Dazai surveys the room and finds indications of a recent presence. As he approaches the communicator, a large foreign man looms behind him and asks his purpose for coming to the room. Dazai fakes a conversation to warn him that the Armed Detective Agency has found their hideout and demands to see their boss. The big man quips that he does not know Dazai, but the latter informs him that he works undercover for the boss. The man acknowledges this and heads out to get the boss, upon which Dazai breaks a liquor bottle to the man's head, causing him to fall to the ground.

Battle
Turning back to the communicator, Dazai tries to input the final command to effectively stop the transmitter signal paralyzing the airplane, only to deduce that he needs a release key to change the settings. Suddenly, from behind, the big man punches Dazai and sends him colliding with a desk. As Dazai grins and mutters how his punch hurt, he immediately kicks off the desk to evade the big man's punch reinforced with some hammer-like steel knuckles. Dazai teases the man to get into a freight delivery business.

Dazai admits his weakness compared to the big man, who blocks the communicator from Dazai. The latter then opts to run out of the room and instantly shuts it close on his way out. Once the big man reaches out to the door, Dazai drop-kicks it from the other side, hitting the man and sending him flying back. Dazai marvels at the strike he committed, then he approaches the man to follow up with another hit, only for the latter to sweep him off unfazed from his earlier attack. Dazai manages to evade the kick and acknowledges the man's toughness.

As the big man gets back up and throws a hook, Dazai bobs and weaves to evade it, but part of his clothing gets caught in the man's knuckles that pulls him off-balance. The man lands punches at Dazai regardless of his attempts to dodge, sending him flying across the room. He then throws a backhand at Dazai, which makes him conclude that he cannot beat the big man. He glances out the window and sees Kunikida fighting an ability user. Dazai receives another punch powerful enough to shatter the window on impact.

When Dazai locks eyes with Kunikida from below, the two call out to each other, having apparently exchanged thoughts at the same instance. Kunikida pulls out a wire gun on his waist and shoots at Dazai's direction, upon which Dazai instantly kicks off the window frame and leaps out the window. Dazai then switches positions with Kunikida, who glides through the air by utilizing the tensile force on the wire of his wire gun.

Aftermath
Upon landing on the first floor, Dazai casually walks over to Kunikida's opponent, a young man who can manipulate people he marks numbers with. The young man becomes livid that his ability is not taking effect on him, with which Dazai informs him that abilities do not work on him. The young man desperately asks who he is, knowing that his entire history has been wiped clean. Dazai lands a strong punch at him and renders him unconscious, afterwards introducing himself as Osamu Dazai, an Armed Detective Agency member.

Dazai then regroups with Kunikida at the operations room and panically shows the broken release key to Kunikida. The latter appeases his concern and takes out an emergency spare page in his hip pocket. Kunikida then replicates the release key, which greatly astonishes Dazai.

After Kunikida presses the disable button to return the airplane's controls, Dazai calls the pilot, though they find the airplane at a relatively low altitude. The pilot eventually manages to regain the plane's altitude, resulting to Dazai's and Kunikida's mission success.

Light Novel and Anime Differences
Since the events in the anime take place after his entrance to the Agency, Atsushi Nakajima accompanies Dazai and Kunikida to the military base. He and Dazai then head to the operations room, where Atsushi aids Dazai in battling the big man. Despite their efforts, they struggle against the big man, until Kunikida switches places with Dazai and defeats the big man in apparently a single counterattack with his physical prowess.

Generally, the battle is shortened in the anime, omitting some scenes from the light novel. Notably, while the light novel depicts him having some minor successes in fending off and occasionally tricking his opponent, Dazai (along with Atsushi) is seen visibly challenged fighting the big man in the anime, even soon being strangled by the glass window.