Yukichi Fukuzawa and Ōgai Mori vs. Illegal Immigrant Group

Yukichi Fukuzawa and Ōgai Mori vs. an illegal immigrant group is a twofold battle fought between former professional bodyguard Yukichi Fukuzawa and Yokohama underground doctor Ōgai Mori against an illegal immigrant group of outlaws.

It first happened inside Mori's clinic and later in the outlaws' base during the post-war era when the Port Mafia's erstwhile boss has been causing chaos and violence throughout Yokohama. The entire plot is apparently foreseen by Sōseki Natsume as Mori later reveals.

Prologue
Roughly two years after the Great War, Fukuzawa was appointed by Natsume to guard the underground doctor Mori and meets him in his clinic. After exchanging quick banters for introductions, Fukuzawa accepts to help out in his clinic. Later, Mori receives a shot patient and opts to treat the same man who injured said patient. Mori assures Fukuzawa that the underworld's stories would never bore him in the neutral-zone clinic, though Fukuzawa grows suspicious of him.

First Encounter
Mori is later beaten up and interrogated by three outlaws given how his clinic has received various kinds of information from his criminal clientele. They ask him the location of the Port Mafia's arsenal; with the Mafia boss's recent rampages throughout Yokohama, the group plans to pin their crimes on the Mafia using their own weapons. Seeing that the outlaws aim to cause further disaster in the city, Mori retorts that, though he likes the city and its darkest parts, he hates parasites like them.

He begs the question to Fukuzawa behind them, having killed off the 20 guarding outlaws outside the clinic. The outlaw threatens to kill Mori, but Fukuzawa does not really mind. After all, he knows that Mori was merely biding some time until he arrives. Using his hidden knife, Mori slashes the outlaws and admits that he pretended to get caught to find out where their base is. He also mentions how everything has been going according to Natsume's plan, much to Fukuzawa's confusion.

Counterattack

 *  Note: The details from the beginning of this battle are not shown. 

Fukuzawa and Mori arrive at a Western-style mansion, having later defeated the various armed outlaws by themselves. Notably, Fukuzawa is hinted to have done practically "half the job" in fighting off their opponents as Mori stated, much to Fukuzawa's irritation at how there were a dozen more foes than what Mori had originally estimated.

Aftermath
Fukuzawa notions how pointless flushing out a minor conspiracy is compared to the Mafia's violence, noting that many more disasters will happen as long as the Mafia continues at the root. Mori assures that he is aware of such situation, which is the reason he accepted Natsume's Tripartite Framework that aims to preserve the peace and order of Yokohama among three constituents.

Manga and Anime Differences
As a minor difference, the manga portrays Mori, tied up, complaining of an itch on his cheek that he asks his captors to scratch; an outlaw then punches him in the face, which Mori gets satisfied with. This is omitted in the anime. Moreover, in the manga, Fukuzawa remarks that he can sense Mori's bloodlust through his hidden knife. The anime phrases it in a different manner, as Fukuzawa recognizes that he and Mori share the same "disgusting stench of death".