To enjoy Samurai Spy you'll find three things are helpful. First, some knowledge of Japanese history. Second, the ability to keep straight a lot of Japanese names, some of which sound alike to Western ears, such as Tatewaki and Takatani. And third, a fondness for pulp adventure stories.
So, history first...and take notes because this is important to what the story is all about. Hideyoshi Toyotomi had succeeded in unifying Japan, but he spent a lot of money and time in two brutal invasions of Korea. When he died he left unrest and a young heir, Hideyori. Ieyasu Tokugawa was a noted general, a leading supporter of Hideyoshi and one of the regents for Hideyori. He also was smart and ambitious. He managed to maneuver things in such a way that he could not be blamed when his forces and the forces supporting Hideyoshi's young son came to blows. In 1600 Tokugawa decisively beat his enemies in the great battle of Sekigahara. But now the forces of the Toyotomi, based in Osaka, are gathering their strength again. Tokugawa, shogun since Sekigahara and based in Edo, will not tolerate this and is gathering his forces. And both sides are employing ruthless spies.
Now all those names. Sarutobi Sasuki (Koji Takahashi) had been a spy for a clan allied with the Toyotomi. Now he is sick of war and has left all that behind. He is a strong man of noble character. Tatewaki Koriyama (Eiji Okada) had been a key spy for the Tokugawa but has defected to the Toyotomi. Sakon Takatani (Tetsuo Tamba) is the main spy for the Tokugawa and is determined to find and destroy Koriyama. Takatani is a master swordsman and skilled in all the ways of the spy.
The story is a pulp samurai mystery-adventure. The conflicts of good and evil, of the horrors of war and the desire for a peaceful life, are played out -- and talked about a lot -- by characters who are either brave and good or who are really bad. We have Sarutobi Sasuki, drawn in against his will to help protect and defend weaker men and women. He, too, is a master swordsman as well as highly skilled with the shuriken, the throwing star. The relentless Sakon Takatani is always dressed in white, with a white turban-like head-covering that allows us only to see his face. He can leap from high bridges and never sprain an ankle.
The search for Tatewaki Koriyama puts everyone into a boiling political stew of betrayals, murder, kidnappings, torture, family secrets, even a bit of leprosy. This is pulp fiction, but good pulp fiction. Most people probably will enjoy Samurai Spy, and most probably won't remember much of it a year later. But like a good Jim Thompson tale, this just means you can enjoy it again almost as much as you did the first time.
The Criterion DVD, one of those in Criterion's Rebel Samurai four-movie set, looks just fine. The extras include a video interview with the director and a gallery of key characters. I'd suggest you study the gallery before you watch the movie. The DVD case includes a brochure with a substantial essay on samurai movies and Masahiro Shinoda, the movie’s director, by a fellow named Alain Silver. While interesting, the essay seemed to me to take the film and the samurai genre far too seriously. There are many excellent samurai films. Samurai Spy, for me, however, is simply a good rouser with plenty of improbable action.
And one last history lesson. By 1616 Ieyasu Tokugawa finally eliminated his Toyotomi problem through a campaign of siege and trickery. Hideyori Toyotomi performed seppuku at 22. The Tokugawa shogunate lasted for 250 years in a time capsule. Then, even Tom Cruise couldn't save the samurai way after Commodore Perry sailed into Tokyo Bay.