Japan's Sengoku period, the 1500s.

'Kokusha,' an assassin group formed to resist the tyranny of the Japanese Imperial Family, who hypocritically manipulate the nation at their whim.

Kokusha has spent a long time eliminating high-ranking imperial figures, eventually rising to become the Imperial Family's top priority threat.

At the center of it all stands Kurogane Saya, the leader who founded and has led Kokusha to this day.

One day, Saya receives classified information that the troops guarding the residence of Prince Guest have been largely moved to suppress a rebellion.