Atsushi Nakajima and Ryūnosuke Akutagawa vs. Ōchi Fukuchi

Atsushi Nakajima and Ryūnosuke Akutagawa vs. Ōchi Fukuchi is a battle fought between Armed Detective Agency member Atsushi Nakajima and Port Mafia member Ryūnosuke Akutagawa against the Decay of the Angel leader Ōchi Fukuchi, otherwise known as Kamui.

It happened aboard the special passenger ship Boswellian during the meeting of various international VIPs to address the creation of a global military anti-terrorist corps that shall be prospectively headed by the war hero and Hunting Dogs commander Fukuchi.

Prologue
Atsushi accompanies Ranpo Edogawa to Standard Island in order to seek Fukuchi's aid in proving the Agency's innocence as advised by Yukichi Fukuzawa. While Ranpo and Fukuchi reunite amicably, things headed to a sharp turnaround when Ranpo activated Super Deduction and realized that the Decay of the Angel's boss, Kamui, is Fukuchi himself. Hence, Ranpo sets off a signal and soon gets slashed by Fukuchi, albeit he manages to get safely to one of Edgar Allan Poe's safe novels. Atsushi then resolves to face Kamui to end the terrorism once and for all.

Meanwhile, Akutagawa turns out to have been tailing Atsushi on orders of Osamu Dazai two days before getting arrested and detained. As such, he notices the smoke signal lit by Ranpo and muses how the prediction seems to be true after all.

Battle
Atsushi readies his stance to fight Fukuchi, but the latter points out that his right leg has already been severed. Fukuchi also notes that he must not be surprised, given that he knows about his weapon-enhancing ability when Ranpo deduced earlier. While Atsushi's leg starts to regenerate, Fukuchi notions to cut off his head, a thought that plunges Atsushi into solitary fear.

Akutagawa then intervenes and quips that the mantiger only ends up as a more bothersome opponent the more he gets pushed to adverse instances. Having no intention of revealing Kamui's identity, the mafioso instead plans on killing him. Akutagawa instantly unleashes Rashomon in a quick attack; he also reveals having destroyed the engine room to halt their travel on sea and maximize their time to defeat Fukuchi.

A weary Fukuchi then accepts their invitation to battle, crumpling the ship's handrail with his bare hands. Akutagawa then clads himself with his Demonic Armor and gives Atsushi 30 seconds to fully regenerate. While Akutagawa rushes at Fukuchi, the latter swings his weapon and slices off the ship's ; Akutagawa barely evades the attack thanks to Atsushi's warning.

Fukuchi once again launches an attack with his handrail, which Akutagawa blocks by channeling Demonic Jaws in his arms. He also manages to break it, afterwards rushing at Fukuchi swiftly to claw his head. However, Fukuchi flips him with his martial arts prowess and gets him cornered at the wall. As Fukuchi approaches him with his brandished blade, Akutagawa defends by drawing his arms back while leaving his Demonic Armor in place. The next instant that Fukuchi attacks, a fully-regenerated Atsushi saves Akutagawa, bringing him to retreat inside the ship.

Akutagawa berates Atsushi for running from their enemy, but Atsushi reasons out that they are no match for Fukuchi. The two argue once more, before a ship guard arrives and sees them. Akutagawa ends up incapacitating the man as part of upholding Atsushi's bargain of not killing anyone within six months.

Akutagawa soon proposes using Beast Beneath the Moonlight–Rashōmon: Black Tiger's Supreme Claw like before, but they admit that Fukuchi is far smaller than Ivan Goncharov's stone giant. Nonetheless, they agree that using their combined powers is the optimal move to defeat Fukuchi. They also strategize to keep Fukuchi underwater to limit his mobility, attack range, and the resources he can weaponize. To do so, they plan to use a VIP submarine as diversion and attack Fukuchi from behind.

However, they soon face Fukuchi having waited by the submarine port, having anticipated their tactic as well. After Fukuchi narrates his story that molded his thinking of bringing down all nations to end all wars, Atsushi and Akutagawa activate their respective abilities and charge at Fukuchi.

Fukuchi blocks their simultaneous attacks, during which Akutagawa aims to attack with Demonic Jaws. However, Fukuchi grabs Atsushi's tiger arm and uses it to punch into Akutagawa's forearm, which breaks his Demonic Armor. He then kicks the mafioso and throws a stone towards Atsushi, both inflicting heavy damage on the two. Atsushi soon realizes that their undercover strategy is debunked, since Fukuchi's enhancing ability may still have a long range regardless of the space they are in.

Akutagawa forces his fracture in place with Rashōmon, while Atsushi stands back up. Discarding his blade, Fukuchi summons Shintō Amenogozen into thin air, also known as the space–time sword. As Atsushi merely begins to marvel and wonder about the blue sword, Akutagawa collapses after getting apparently stabbed. Atsushi soon gets impaled from behind by Shintō Amenogozen, which passed through space.

Fukuchi ends the battle in his proclaimed victory, telling the two that they had no trust in each other in the first place. Had Atsushi been the vanguard with Akutagawa as the rearguard, Fukuchi supposes that they might have had a chance to take his head. Atsushi admits that, though it might be Dazai's logic, he would not do something reckless. Meanwhile, Akutagawa chuckles at the thought of possibly gaining Dazai's acknowledgement if they defeat the undefeated hero Fukuchi.

Rather than personally breaking him, Fukuchi invites Akutagawa to become his pupil and groom him into becoming the strongest Hunting Dog ability user, after which he is to return him to Dazai. Akutagawa rejects his offer, but Fukuchi expounds that he made such deal out of his plan to have Akutagawa kill Atsushi instead of doing it himself. Fukuchi further persuades Akutagawa that he had no motive for killing Kamui and questions whether he finds Atsushi's life significant.

Akutagawa then stands up and glares darkly at Atsushi. Atsushi wonders if Akutagawa thinks highly of his life, to which the mafioso responds that his time is running out due to a lung disease, which explains why he is keen to earn Dazai's praise as soon as possible. Atsushi then stands up, and the two charge at each other.

In truth, the two switch positions, Akutagawa clads Atsushi in Rashōmon, and Atsushi successfully cuts off Fukuchi's arms with Black Tiger's Supreme Claw. Atsushi asks Fukuchi not to die yet to announce the Agency's innocence, but Fukuchi instantly succumbs to his demise.

Atsushi swipes the page from Fukuchi, and Akutagawa declares their mission to be successful, motioning that they escape as soon as possible. Atsushi appears sympathetic towards Fukuchi's last words, to which Akutagawa notes that his sword has already been cut off, possibly done by the Black Tiger's Claw. However, Atsushi realizes that Shintō Amenogozen has been cutting into something at the last moment of their surprise attack, arousing his suspicions within the situation.

Time soon rewinds to the past, with Akutagawa and Atsushi switching places for their surprise attack. However, Fukuchi cuts off Atsushi's right arm and slashes the defenseless Akutagawa. Fukuchi had apparently cut Shintō Amenogozen into the past, a strategy Atsushi finds to seal away their victory right from the start.

Fukuchi asks Akutagawa for his final words, to which Akutagawa retorts that he needs only actions over words. He then uses his remaining shirt into his ability and cuts through the ship to acquire an extinguisher, after which Akutagawa and Atsushi escape through the smoke. However, Fukuchi cuts through time once more and breaks the extinguisher, preventing Akutagawa and Atsushi from escaping. Fukuchi then praises Akutagawa's final words and slashes through his neck.

In his final moments, Akutagawa instructs Atsushi to flee, with which he escapes through the VIP submarine. Fukuchi then plans on chasing him, but the ship guard earlier and various others arrive on deck to aid in catching the terrorists. Fukuchi refuses their offer, showing that he has now killed Akutagawa, and merely asks them to chase down the other one.

Aftermath
Before Akutagawa's corpse, Fukuchi brings forth and opens a coffin, revealing Bram Stoker inside. The fifth Decay member and an being, Bram is asked by Fukuchi to bite and turn Akutagawa into his kind. He initially declines but gets threatened to do so otherwise and ends up infecting Akutagawa. From then on, the Decay of the Angel launches its fourth phase into the world, which involves a vampire infection outbreak that starts from the Mafia.

Meanwhile, Atsushi manages to break out of the submarine and swim away. Ango Sakaguchi rescues him and listens to his story aboard a passenger boat. Atsushi then muses on Akutagawa's last stand and how him upholding his part of their six-month deal allowed him to survive and reveal the truth to Ango. He ultimately wonders whether there was something important behind it or merely coincidence.

Trivia

 * Thus far, this battle is the longest one in terms of chapter count, spanning a total of six chapters.