Kenji Miyazawa and Akiko Yosano vs. Chūya Nakahara

Kenji Miyazawa and Akiko Yosano vs. Chūya Nakahara is a battle fought between Armed Detective Agency members Kenji Miyazawa and Akiko Yosano against Port Mafia executive Chūya Nakahara during the three-way gifted organizational conflict.

It happened at an abandoned underground railway tunnel that serves as the entrance to the Agency's hideout during the Mafia's attempt to lure the Agency into engaging the Guild in combat.

Prologue
As the Agency, the Mafia, and the Guild mutually declare war against one another, the Agency is compelled to transfer their base of operations from their office to the Bankoudou Hall. Meanwhile, after the Guild intercepted the earlier standoff between the Agency and the Mafia that resulted to Kōyō Ozaki's capture by the Agency, Mori opts to hire killers to ambush and assassinate Fukuzawa.

However, Fukuzawa easily overwhelms the assassins with his martial arts prowess. Nevertheless, as per Mori's plan, the assassins' sleeves are planted with a radioactive scandium marker, which – after Fukuzawa came in contact with it – allows Mori and Chūya Nakahara to track down Fukuzawa and, by extension, the Agency's hideout. After Chūya traces the Agency's hideout near the underground railway, Ranpo Edogawa takes notice him and notifies Fukuzawa.

The President instructs them to activate the tunnel's automatic guns to fire at the Mafia executive, but Chūya easily destroys them together with the surveillance cameras and challenges the Agency members to face him directly rather than their inanimate weapons. Hence, Fukuzawa sends in Kenji Miyazawa and Akiko Yosano to meet the Mafia's envoy.

Since the detectives correctly deduce that Chūya has not come to tear down their hideout, Yosano directs him to a camera to communicate with Fukuzawa. Chūya presents him with a recent photo and the whereabouts of John Steinbeck and Howard Phillips Lovecraft, having lured the Guild members in with a bait. He then persuades the Agency to ambush them on such rare chance. To give the President time to deliberate, Yosano proposes to spill Chūya's intentions by force in combat, to which the mafioso eagerly agrees.

Battle
Kenji rips out a rail from the ground with his strength and swings it towards Chūya, but the latter evades the attack and lands upon the rail afterwards. Chūya then rushes at Kenji and unleashes a powerful kick; Kenji blocks him with both arms but is sent flying by the force of the attack. Yosano instantly sneaks behind Chūya with her hatchet, but when she swings it towards Chūya, he quickly dodges by standing upon the tunnel's ceiling.

Yosano recognizes him as the gravity manipulator Chūya Nakahara, much to the latter's annoyance that Osamu Dazai must have told them. Within the rubble, Kenji is amazed that Chūya's hat does not fall off even when he is upside down. Chūya then drops to the ground, the impact causing a crevice on the ground between him and the detectives. Chūya challenges them to a fight against gravity, hence causing a stalemate between the two parties.

Aftermath
Fukuzawa interrupts and questions Chūya what the Mafia's next move would be if ever the Agency indeed wipes out the Guild operatives without the Mafia having to take action themselves, still suspicious that he is hiding something behind such enticing proposal. Ranpo soon deduces that the Mafia must have used quite an appealing bait that the Guild will fall for regardless of how it appears like a trap. The mafioso admits that they leaked their clerks' location to the Guild, forcing Fukuzawa to send Doppo Kunikida and Jun'ichirō Tanizaki to directly face the Guild.

Manga and Anime Differences
In the manga, upon Chūya's arrival at the railway, Yosano notes that their second and fifth cameras are down. In the anime, she remarks that their third, fourth, and eighth cameras are wiped out. Afterwards, Fukuzawa instructs them to activate the automatic assault guns, which appear to be latched onto the ceiling in the manga but perched on tactical tripods on the ground that also emit red laser sights in the anime.