The Yamainu play a major role during the events of Matsuribayashi-hen when, early on the day of Watanagashi, Furude Rika is reported to be dead. As this throws Takano's plans of starting the GHD astray, she realizes that the report is faked. The Yamainu 's spies in the prefectural police department attempt to confirm or deny this; however, they are stopped by Ōishi Kuraudo. Despite the report never being found out to be fake, Takano initiates heavy surveillance of Rika's house with about four Yamainu observers. During this time, the Yamainu capture Tomitake Jirō from a hotel in Shishibone City and bring him back to the Irie Clinic while Takano interrogates Irie.
While questioning Irie, Takano becomes increasingly frustrated with a logical impossibility presented to her: before Watanagashi, Rika's house was under surveillance by the Yamainu, who confirmed no one had entered or left the residence besides Irie. Irie claimed that Rika was in the house with a cold; yet, her body turned up at the Okinomiya police station. After a call from Nomura, Takano orders the Yamainu to place Irie under heavy surveillance.
Irie notices that Tomitake was captured and escapes his clinic by car. A group of Yamainu pursue him in one of their vans along the forest road. One of them successfully shoots Irie's tire. When they attempt to secure Irie, they are stopped by Tatsuyoshi Kasai with Sonozaki Shion, but are able to track them based on a tracker hidden on Irie. They pursue them to the Sonozaki manor.
Once Irie, Kasai and Shion arrive at the Sonozaki manor, several other backup vans along with Okonogi arrive to secure the estate. Here they make use of plastic explosives to break down a large door to the Sonozaki torture chamber as well as night vision equipment to see in the dark due to having cut the electricity (along with the phone lines in Hinamizawa to prevent contact to the police/the Banken). While invading the chamber they used hand grenades and possibly assault rifles to secure the area. After abducting Rika, some of the men go back into the basement in order to eliminate the rest of the group protecting her, as the few along with Okonogi go to bring Rika back unconscious. A few men are injured as Akasaka Mamoru rescues Rika and nearly incapacitates Okonogi. The forces retreat while the opposing side sets up a plan to use Irie's tracker bug as a decoy while sending the club members into the mountains with it. The Yamainu are unaware of this plan and subsequently pursue the club with most of their squad, leaving about eight guards in the Irie Institution.
The control center van containing Major Takano and the main radio hub as well as around four other vans head out into the mountains, facing off against the club with taser guns. These prove to be useless against the club's ability to work together and Hōjō Satoko's prowess with traps. Meanwhile, Irie, Kasai, Akasaka and Shion infiltrate the Irie Institution, incapacitate the eight guards, ruin the Control Room's main interface and place Hōjō Satoshi in a safer area for recovery. After Tomitake is released, he and Akasaka escape to Okinomiya after facing off against two Yamainu securing the borders of Hinamizawa armed with RPGs and assault rifles. Tomitake calls in the Banken, another military organization under the control of "Tōkyō" for assistance in keeping the research secret.
Eventually, most of the Yamainu in the mountains are wiped out. Okonogi shows his skills in melee fighting against Mion but is defeated. He considers the entire ordeal to be over, and he reveals to Takano that due to this failed scenario Nomura is going to have her be killed. He tells the rest of the Yamainu to surrender to the incoming Banken while offering a handgun to Takano for the purpose of killing herself. Afterwards, Okonogi surrenders along with the rest of his troops while assuming the entire incident will be wrapped up neatly by "Tōkyō."
