The film opens with an old, broke samurai appearing at a mansion, seeking permission to kill himself, in the ritual suicide of hara kiri (or seppuku, which the film is also known as) to end the shame that his life has fallen into. In a short period of time he is introduced to the lord of the mansion, who tells him of a similar samurai who, some weeks before, had appeared to beg a similar boon. It is not uncommon, the lord says, for poor samurai to appear at a lord's house and threaten hara kiri, in the hope of receiving a pittance, and then leaving. It was believed by the lord and his retainers that the young samurai who appeared here had a similar plan, and so they, in response, devised a plan that will ensure that the young, broke samurai, did in fact commit hara kiri with his own swords. They did it, so he said, to set an example to other samurai who might try to extort money from them. The older samurai, upon hearing this story, assures the lord that he is quite intent on killing himself, and preparations are made. But, before he does commit hara kiri, he tells the story of his life, and his connection to the young samurai.
I'm reluctant to explain much of the plot, since one of the joys of Hara Kiri is in its construction, and the way that Kobayashi weaves the back flashes together with the main plot, and how he manages to take the initial impressions of righteous indignation on part of the lord, and subvert it, leaving it a shallow, and hollow thing by the end of the film. Which, of course, is much the point of the film, because Hara Kiri is not intent on presenting a traditional samurai film, in which honour is elevated to a principle that must be upheld at all time, but rather it seeks to present a view in which honour is used to hide the selfish desires of those in power. It is arguable--and I am not sure, based off Japanese culture or the period of time, since I am no expert--but it is arguable that what Kobayashi has created a film that critiques the representation of honour that is presented by people in power, saying that this honour ignores the social and economic influences on the poor and working class people beneath them.
With that argument in mind, it is impressive to note that not one moment in the film, either in back flash or current, fails to add to the plot, character, and thematic concern of the film. The one moment of fat on the film appears at the end, with the obligatory final fight scene, which I thought, personally, Kobayashi could have cut (however, it can be seen to add to the theme still). But otherwise, each moment serves multiple functions, as the scene in which the elderly samurai (played by Tatsuya Nakadai) is approached by a lord, who seeks to adopt his daughter, but who refuses on the grounds that he believes that it is merely a pretext to making her a concubine. Here, you are introduced to the samurai's selflessness when it comes to his daughter, and the importance of her to him, and his unwillingness to use her to rise his station in the world; you are also introduced to the belief of those in power that young, pretty women should be elevated in the world so that they may serve a lords sexual pleasure.
Hara Kiri is an excellent film, really, on every level. Nakadai's performance is strong, and charismatic, but with a sense of sadness throughout, and he holds the focus of the film well. Those actors around him, are likewise, performing well, and the look and sound of the film come together to create a full, and satisfying whole. In fact, rather than go on, I'll just say that if you haven't seen it, you should do yourself a favour, and track it down.