Asano Naganori is the lord of the Ako clan and every other year he must go to the capital Edo and spend a year living there to cement the peace between the clans.
On arriving in Edo he is given the news that his wife has borne another child and immediately suspects foul play due to the babies emerald green eyes.
The Shogun Tokugawa Tsunayoshi instructs his master of ceremonies Kira Yoshinaka to teach Asano the proper etiquette required as he is to host important guests and the Ako clan have served the Shogun faithfully for generations.
On meeting Kira, Asano is struck by the man's emerald green eyes and all becomes clear to him, his honour must take precedence and he will kill the master of ceremonies.
Faced with twelve samurai, Asano realises that he has dishonoured himself in an effort to maintain his honour he has broken the law twice by drawing steel and killing a guard.
Asano's honour and the samurai mandate require he commit seppuku, take his life, first by driving his sword into his abdomen and then decapitation, a ritual to save further dishonour.
Oishi Kuranosuke the chief retainer of the Ako Clan faces one option, only revenge and the death of Kira Yoshinaka will satisfy their honour and they are now Ronin, the masterless Samurai. On their trip to the capital they encounter several battles which reduce their number from over 300 to just 47 Ronin, but revenge and duty to their late lord must wait for an opportunity to present itself.
A compelling tale of honour and revenge as these warriors were the inspiration behind bringing back 'the way of the warrior' to the hearts of the people. Quite how Keanu Reeves and the magic of a witch fits into this I don't know but the film looks really good and the book definitely is.