It's an amazingly detailed manga about a real-life samurai, Miyamoto Musashi, who is held to be the best Japan has ever seen. He wrote an infamous battle tactics and sword-handling book - the book of five rings - and he is the founder of the modern use of two swords in battle. The manga is essentially an embellished tale of his life, how he refined himself from a hunted, demonized killer into the best swordsman in history. It's really, really, really good.