Watch Donten: Laughing Under the Clouds - Gaiden: Chapter 3 - Conspiracy of the Military - Episode 1

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. (Source: Amazon Prime Video)