The Mimawarigumi was a special police force under the control of the shogunate. They were the main antagonists during the Thorny Arc and remained somewhat antagonistic until the Farewell, Shinsengumi Arc.
Like the Shinsengumi, the Mimawarigumi are tasked with finding and arresting Jouishishi and other terrorists as well as regular police duties. They, like the Shinsengumi, are under the direct control of the Shogunate, meaning that they are higher in rank then the regular police and are sometimes tasked with protecting government officials. They have a notable rivalry with the Shinsengumi which also includes wearing the opposite color scheme, their members and their residence.
Isaburo and Matsudaira decided to form the Shinsengumi four years before the start of the series. When the government saw how successful this force became, they decided to create another police force that is the opposite of the Shinsengumi, made up entirely of high class samurai and retainers. They chose Isaburo, who is himself a samurai noble to lead the group and his retainer, Imai Nobume, as his vice-commander. It is unknown when the group was formed beyond approximately months or a couple years after the Shinsengumi's founding.
It is unknown how many members are in the Mimawarigumi but only two are named and recurring.
The Mimawarigumi are introduced getting involved in a confrontation between Isaburo's half-brother, Sasaki Tetsunosuke and Sakata Gintoki with Hijikata Toushirou caught in the middle. Two Shinsengumi members later talk about this police force's origins and rise in influence. Tetsunosuke was then kidnapped by a Joui faction called the Check It Out Gang and both forces arrive to capture or kill them. It is revealed that Tetsu was a former member of the Gang before fleeing and it is implied that Isaburo sent his half-brother to the Shinsengumi to disgrace them for harboring a terrorist and getting rid of his bastard brother. Hijikata, who is sympathetic to Tetsu due to their similar backgrounds, fights Isaburo, while Kondou Isao, Yamazaki Sagaru, and Okita Sougo try to reach the gang on the roof before the assassin vice-chief Nobume and some of the Mimawarigumi. Nobume ends up fighting Okita due to their similarities while Kondou, Yamazaki and later arrival Hijikata save the gang and Tetsu due to Gintoki (who was released after his arrest then hired by Isaburo to infiltrate the gang). The Shinsengumi forces Isaburo to take the gang in alive while not revealing Tetsu's involvement with the gang and their fight. The dismissive Isaburo gains some respect for the Shinsengumi.
It is later revealed that Isaburo has ties to Takasugi Shinsuke.
The Mimawarigumi was in charge of being guards to the shogun's palace due to a string of government official killings. The shogun's uncle, Tokugawa Sada Sada, reveals to the disguised Tsukuyo, Gintoki and Nobume that the Mimawarigumi had ties to a political faction within the government called the Hitotsubashi Faction. The murdered officials were Hitotsubashi and the suspicion has fallen on the Tokugawa Faction. At the same time, Isaburo gives the same information to Shimura Shinpachi and Kagura and warns the teens about Sada Sada's ruthlessness. Unfortunately, Sada Sada arrests Tsukuyo, the Yorozuya, and Nobume, framing them for the killings that he was truly behind, while heavily wounding Isaburo to get rid of the Mimawarigumi.
The Mimawarigumi were replaced by the Shinsengumi, who after learning the "criminals"' identities and the real circumstances about what is happening, purposely freed them to confront Sada Sada. As Nobume fights with the Yorozuya and Tsukuyo, the still injured Isaburo was asked by Shige Shige to work with the Shinsengumi to protect the five from Sada Sada's enforcers, the Tenshouin Naraku. They all succeed in bringing down Sada Sada. Isaburo returns to the hospital and is congratulated by a Hitotsubashi member for helping to weaken the Tokugawa Faction's position. He is later confronted by Kondou and Hijikata about the member's statement, but he admits his surprise that Gintoki's influence to these events had changed everything, including Nobume.
It is revealed that Nobume was a former Naraku.
Kondou, Hijikata, Okita, Isaburo, and Nobume visit a lecture by Matsudaira about continuing to keep alive the knowledge gained from working in the field along with other high ranking inspectors. The five are forced to participate in a virtual reality simulation of trying to solve an old case thanks to Hijikata and Isaburo's argument. Although the five compete to find what was the cause of death, they fail.