47 Ronin and the Shadow Ninja

In Genroku-era Japan, after Lord Asano is forced to commit suicide for attacking Kira, Chief Retainer Araki sends Kaizuka Shingo undercover to investigate Oishi Kuranosuke's motives. As Kira's son deploys spies to stop Oishi's revenge, Kaizuka joins the samurai's cause, setting the stage for a historic raid.