"Donten ni Warau Gaiden: Ketsubetsu, Yamainu no Chikai" is a three-part anime OVA series that serves as a prequel to the main "Donten ni Warau" storyline, delving into the backstory of key characters and events that shape the narrative. Set in the Meiji era of Japan, the story explores themes of loyalty, sacrifice, and the clash between tradition and modernization. The OVA focuses on the Yamainu, a secret government organization tasked with capturing and imprisoning criminals in a secluded floating prison known as Gokumonjo.

The central characters are the Kumoh brothers: Tenka, Soramaru, and Chutaro, who are descendants of the legendary hero Yamato Takeru. Tenka, the eldest, is a carefree and seemingly irresponsible individual who hides his deep sense of duty and protective nature. Soramaru, the middle brother, is serious and driven, often struggling with feelings of inadequacy compared to Tenka. Chutaro, the youngest, is cheerful and innocent, providing a contrast to his older siblings. The brothers are drawn into the conflict surrounding the Yamainu and their mission to maintain order in a rapidly changing society.

The plot revolves around the Yamainu's efforts to capture a group of rebels led by Shirasu Kinjo, a former member of the organization who seeks to overthrow the government. Shirasu's motivations are rooted in a desire to avenge the injustices he perceives within the system, creating a moral gray area that challenges the Yamainu's sense of duty. The story also explores the relationship between Tenka and Abe no Hirari, a skilled swordswoman and member of the Yamainu, whose loyalty and personal struggles add depth to the narrative.

As the conflict escalates, the Kumoh brothers are forced to confront their own beliefs and the sacrifices required to protect their loved ones and uphold justice. The OVA provides insight into the origins of the Yamainu, the bonds between the characters, and the events that set the stage for the main series. The narrative is driven by a mix of action, emotional drama, and historical intrigue, offering a compelling exploration of the characters' motivations and the societal tensions of the era.