Donten ni Warau Gaiden: Ouka, Tenbou no Kakehashi

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)