47 Ronin and the Shadow Ninja(1981)

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 Kiras son deploys spies to stop Oishis revenge, Kaizuka joins the samurai's cause, setting the stage for a historic raid.