In fact, there were 47 ronin. They were retainers of Lord Asano, who, in 1701, was asked by the shogunate to serve as a representative when imperial envoys came to Edo. Asano was to be trained by Lord Kira, who expected to be bribed. Asano would not bribe Kira, and Kira arrogantly refused to teach Asano what he needed to know. Provoked beyond endurance, Asano drew his sword in the shogun's palace and attacked Kira, who did not fight back.
Drawing a sword in the palace was a capital crime and Asano was ordered to commit suicide and his fief confiscated--making all of his retainers ronin, masterless samurai. Forty-seven of them, convinced the tragedy occurred because of Kira's arrogance, vowed to avenge their master's death. They dispersed, dissembled, and waited almost two years to convince both Kira's and the shogun's spies that they were resigned to the situation. On the last day of January 1703, they attacked Kira's heavily guarded residence, found him hiding in a toilet, and killed him. They then marched five miles to the site of Lord Asano's grave to present Kira's head.
The situation presented the government with a dilemma. On the one hand, the ronin had demonstrated loyalty to their master—a major value in the society. On the other, they had violated the law of the land. Private morality or public law? Although the public widely supported the action of the ronin, they were ordered to commit suicide. They did so and you can visit their graves in Tokyo today, as I have done.
Within three years, there was a puppet play based on the incident (Chushingura, or The Treasury of Loyal Retainers) and in the modern era more than 100 novels and movies have used the story. Now Universal Pictures has made something called 47 Ronin with a sword-wielding Keanu Reeves.
Because I have not seen the movie, only the trailer (which is available from the Variety site), I cannot judge the movie fairly. Perhaps the story is closer to the original that the trailer suggests. Although having a Westerner play the leader of the 47 is a little bit like having a Japanese actor lead the defense of the Alamo. Perhaps there is room in the story for women turning into monsters, horsemen galloping across the landscape, helmets that resemble skulls, explosions of fire, CGI animals, and more. Perhaps.
But I suspect the profound questions of private morality or public law got lost in the special effects. I'll be interested in the reviews when the movie opens in America on Christmas Day.