Baian is based on a series of short stories by author Shotaro Ikenami. They're about an 18th-century acupuncturist named Fujieda Baian who subsidizes his charitable medical services to the poor by using the same skills as a paid assassin to take out villainous types.
Baian the Assassin: Vol. 1 (1973) is the first feature film and deals with two assassins who are hired to carry out a hit on the second wife of a candle merchant.