The Sword of Hayato, a full-time Edo Retainer

Shioya Hayato served as a chief retainer at the Edo residence of the Amagasaki Domain of Settsu for over twenty years, and his name was known throughout Edo for his wisdom and bravery. He was also the domain's greatest swordsman, and his school of swordsmanship was one of the ancient styles of swordsmanship known as the Nakajo school.
At the age of sixty, he applied to retire, but his request was rejected on the grounds that someone was plotting to take over the domain.
Who is the enemy that stands in Hayato's way as he risks his own life to save the domain and its people?