It is clear from the first few frames of Samurai Spy that we are about to watch a film that stands an arm's length away from its peers in the chanbara genre. It begins and ends with a narrator explaining the story's historical backdrop, immediately making it clear that the Battle of Sekigahara, which took place in 1600, serves as the catalyst for the events that are to follow. The characters' lives have been shaped by it to such an extent that some see themselves as mere riders on the crest of a wave of war; they have enjoyed relative peacetime for nearly 14 years, but can sense that it will soon pass.
Like Sword of the Beast, the film was released in 1965, and has since become part of the Criterion Collection's Sixties Swordplay Classics. (This strikes me as a bit odd, considering that, while Beast balanced the internal and external action, Spy more clearly focuses on the former.) Another, more defining, commonality is that this is also a character-driven story with a complex protagonist at its core.
Our hero in this case is a samurai named Sasuke, who is, in his own words, "pursued, always pursued by something." He is trying to literally run from the violence that has made his life the way it is, and somehow avoid the conflict he and everyone else knows is inevitable. Above all else, he wishes that the peace he has gotten used to will last, and is thus reluctant to engage in the battle and intrigue that thrust Spy's plot forward, despite his exceptional skill with a sword. One of his final opponents, bleeding to death after being felled by Sasuke, puts it best: "You're a strange man," he says, moments from death, "you truly are."
Sasuke's main obstacle, then, appears to be an enigmatic spy, dressed all in white and with most of his face covered, who manages to show up at all the wrong times. But there's more, much more; in fact, the plot is actually rather trying at points. The cast of characters is long, their allegiances sometimes unknown. To say this detracts from the film is a bit shallow, but it does tend to interrupt the flow. Sasuke finds himself an unwilling participant in this web of intrigue, and his main goal is to spin himself free of it. The first time we see him, he's running through a dense fog, and this is how he spends much of his time: confused and on the run.