Atsushi Nakajima vs. Ryūnosuke Akutagawa (Moby Dick)

The third battle of Atsushi Nakajima vs. Ryunosuke Akutagawa is a skirmish fought by Armed Detective Agency member Atsushi Nakajima and Port Mafia member Ryunosuke Akutagawa during the three-way gifted organizational conflict.

It happened during Atsushi's solo infiltration in the Guild's aerial fortress, Moby Dick, to confront Francis Scott Key Fitzgerald. Meanwhile, after finally locating Atsushi ever since his defeat on the cargo ship, Akutagawa infiltrates the ship on his own personal vendetta of killing him once and for all amidst the erstwhile ceasefire between the Agency and the Mafia.

Prologue
After Ranpo Edogawa and Akiko Yosano acquire intel on the Guild's airborne ship from Edgar Allan Poe, Ranpo and Osamu Dazai arrange for Atsushi to solo infiltrate Moby Dick due to his combat-type ability, his relative familiarity with the ship's terrain following his capture, and his chances of being merely captured if he were to get caught. Atsushi agrees to the mission, and Jun'ichirō Tanizaki aids him to sneak aboard Moby Dick by camouflaging their aircraft and enter the airship at the same time that the Guild's supply helicopter arrives to unload into Moby Dick.

However, the supply helicopter has since been intercepted by Akutagawa, who has killed its pilot and passengers and thereby arrives in the airship. This soon prompts Ōgai Mori to contact Dazai and inform him about Akutagawa's move that breaches the non-interference agreement between their organizations. Eventually, Atsushi and Akutagawa encounter each other along Moby Dick's emptied premises.

Battle
The two hostilely engage each other immediately; Atsushi rushes towards him with his tiger speed, but Akutagawa quickly puts up his defense against his punch, an improvement that Atsushi takes notice of. As Akutagawa launches his tendrils in return, Atsushi dodges and strikes back with his acrobatic maneuvers, which the mafioso acknowledges he has improved on.

Atsushi asks his motive for being in the ship, to which Akutagawa replies that he has come to kill him. He reveals having been tracking him down ever since his defeat on the cargo ship, which has proved to be challenging due to the Agency's covert operations since the outbreak of the three-way conflict. It is only when the Mafia has "allied" itself with the Agency that Akutagawa is able to finally track Atsushi's location by accessing their mission plans. Thinking that Akutagawa wishes to take revenge from the humiliation of his defeat, Atsushi deems his motive to be quite irrational, especially that Moby Dick is on the verge of crashing to Yokohama.

Dazai catches wind of the situation and instructs Atsushi to heed his instructions to evade Akutagawa. After conversing through his communicator, Atsushi tells Akutagawa that Dazai wishes to speak with him. Per Dazai's direction, he immediately tosses the communicator out of the way, and, as Akutagawa jumps off the floor to catch the communicator, Atsushi departs quickly. Akutagawa manages to catch the communicator, only to find that Dazai has hung up on him.

Aftermath
With Akutagawa seemingly out of his way, Atsushi arrives at the ship's top-floor office to retrieve the control terminal that can stop the descent of Moby Dick, where he discovers it to be personally guarded by Fitzgerald himself.

Manga and Anime Differences
The anime shortens Atsushi's and Akutagawa's conversation, not mentioning how Akutagawa manages to track him down. Furthermore, in the anime, rather than point out that Moby Dick is on the verge of crashing to Yokohama during the mafioso's desire for his revenge, Atsushi gives him a general statement of their situation, merely stating that they are on the brink of losing the place they live in.