Watch Donten ni Warau Gaiden: Ketsubetsu, Yamainu no Chikai Online

Fuuma ninja clan is a secret ninja tribe that is waiting for the "Orochi" (gigantic snake) to resurrect for generations. Shirasu and his twin brother Ichiu grew up in this viscous environment and this tells how they become the leaders of the clan.

The three brothers discover that the human experiment banned after destroying the "Orochi", is still going on to secretly revive the evil snake. They also find out that some ex-members of the Yamainu (the Japanese military unit formed to kill the snake) are behind this conspiracy. The brothers fight together again in a battle against the military.

One year has passed after the great battle against the "Orochi" (gigantic snake). Tenka had been paralyzed from the final battle and he took part of the government's human experiment before the battle. Tenka withdrew himself from society, but his younger brothers, Soramaru and Chuutaro, find out the secret past about their older brother.

In the 11th year of the Meiji era: the Japanese population is increasingly dissatisfied with the government due to the increasing speed of Westernisation and the ban on samurai. In order to curb the increased rate of crime, the government opens a new prison. The three brothers of the Kumo family — Chuutarou, Soramaru, and Tenka — are entrusted with the duty of transporting these criminals to the new prison, but adventures await them on the way...