Otōto and Otōtobun

Summary
Fumi and Midori find a samurai-themed keychain on the floor and try to return it to Katō, believing that it belongs to him since he is a ninja and likes period dramas. The keychain, however, turns out to belong to Shinpei - and it isn't long before he and Katō butt heads in a modern-day equivalent of ninja vs. samurai. The two seem incapable of getting along, Katō mocking Shinpei's usage of archaic elevated language when referring to Shinzō and Shinpei mocking Katō's constant fawning over Tsubaki.

Later that day, Katō expresses his frustration while on patrol with Tsubaki, who has problems of his own. Katō soon learns that the reason for Tsubaki's lack of concentration surrounds an incident from a few days prior. Evidently, he had accidentally run into a girl and she now wants him to pay for her doctor's fee, as she sustained a bone fracture from the collision. The fee in question is 10,000 yen, which Tsubaki intends to cover somehow using his own savings as he doesn't want to trouble his father. Katō suggests that the girl's injury was faked, but in the face of Tsubaki's adamance to the contrary, instead decides to talk to the girl himself. When he asks, he finds out that the girl attends Tachikawa High School... which gets him thinking.

Meanwhile, Shinpei has shown up at the Sket Dan's clubroom for a consultation. It turns out that he is worried for his brother, who wholeheartedly fell in love with a girl he met a few days prior. However, the girl is essentially a money leech and Shinpei believes his brother is being tricked. Bossun, having overheard Tsubaki and Katō's talk earlier, notes the similarities between the two incidents and subsequently has an epiphany when Shinpei tells the trio that the girl is from Tachikawa High School. After Shinpei leaves, Bossun tells Himeko and Switch about Tsubaki's incident and how he realized that the two were connected. Himeko panics, believing that Shinpei is walking into an ambush, but Bossun assures her that everything will be fine.

At a Tachikawa High School hangout, Shinpei and Katō have both turned up and almost immediately start arguing again. They soon figure out that the girl in question scammed both Shinzō and Tsubaki, and subsequently team up to beat the delinquents who surround them. As a result, they end up becoming friends.