Sensei begins his long letter by apologizing to the narrator for not replying to his letters and admitting that he, Sensei, was unfairly preoccupied with his own matters. He writes that he will finally tell the narrator about his own life experiences, both out of a sense of obligation to the narrator and also to impart his story to one whom he trusts.
After both of his parents die of typhoid fever, Sensei, then 19 years old, and his sizable inheritance are placed under the care of his uncle. Being naturally generous and trusting, Sensei places complete faith in his uncle, whom his late father had trusted dearly, leaving all matters to him when he goes from his country home to attend university in Tokyo. Though their relations are originally happy, after Sensei refuses marriage to his uncle's daughter, his uncle's family becomes antagonistic to him. Moreover, Sensei later discovers that his uncle has cheated him of much of the inheritance that was properly his, and so he breaks from his uncle, taking the small remainder of his money. Extending his anguished suspicion towards his formerly trusted uncle to humanity in general, Sensei becomes misanthropic.
Despite losing most of his inheritance, the amount he is left with is enough to allow him to look for better accommodation, which he finds in a house where an army widow, her daughter, and their maid live. Over time, Sensei reemerges from his new aloof and suspicious personality, chatting with Okusan, the mother, and Ojosan, the daughter. Furthermore, Sensei falls deeply in love with Ojosan; he does not propose to marry her only out of a suspicion that Okusan is trying to manipulate him into the marriage, as his uncle once did to him. In this way, conflicted between unquestioning love and gnawing doubt, Sensei grows ever closer to Ojosan.
As Sensei remarked, a fateful change occurred when he convinced Okusan to also let Sensei's university friend K. stay with them. Sensei and K. had been friends since childhood, and at least partly because of his background as the motherless son of a priest, K. has an eccentrically single-minded personality. Sensei is himself is quite enamored with this vague but ruthlessly determined attitude of pursuing some "true way," but feels that he is responsible for the often bad consequences of it. For one, K. deceives his foster family about his educational plans, and so when he confesses it, his foster and original family severely rebuke him, and he feels more or less disowned. Now forced to work to pay for his own tuition, K. works himself even harder, relishing in the asceticism of it, but Sensei notices worryingly that his friend is becoming neurotic. In order to help him, he brings him into Okusan's house, hoping that a change of environments will help him as much as it helped Sensei himself.
In the beginning of his letter to the narrator, as he will return to at its end, Sensei remarks upon the two reasons he has for writing about his life story: "Often, I was tempted to abandon the task, and so break my promise to you. But every time I dropped my pen thinking I could not go on, I found that, before a full hour had passed, I was writing once more. You may take this as a manifestation of my naturally strong sense of obligation," and "apart from any sense of obligation, there is the simple reason that I want to write about my past" (127-128). Though it may be a generalization, it would be helpful to notice that the idea of obligation, which Sensei mentions some times in his story, is a very Confucian, traditional Japanese idea. As for the second, individualistic reason of wanting to preserve and tell his own story, Sensei seems to show his Western influence, although one may debate to what extent the concept of "humanity" that he cites and discusses so often is specific to one culture, be it Western or Eastern.
As for Sensei's testament itself, one may notice that the young Sensei was very much like the narrator. Although Sensei often points out cultural differences that have come with the further modernization in the narrator's time, Sensei himself was influenced by many modern ideas and so sees their consequences, as played out in his life, as valuable to the narrator and his generation's knowledge. In other words, although Sensei considers himself a living anachronism, to some extent, at the end of his testament, he believes that his experience, as near enough in historically and, more importantly, as human experience, is still relevant.
Since the purpose of his testament is to explain his life experience to justify the advice he gave to the narrator earlier, Sensei often performs the kind of analysis on himself that he seemed to fear the narrator would do to him: "You wished to cut open my heart and see the blood flow" (128). As he continues to say, it is only because he knows that he will soon die that he is able to think about himself in such an incisive way, implying that it would be very harmful to psychologize living people. Nevertheless, we as readers get the strong impression that both Sensei and also the narrator perform this kind of intense introspection throughout their lives. Compared to the simplemindedness or intuitive bent of most of the traditionally Japanese characters, this tendency is highly modern.
Sensei's love for Ojosan is one of the few seemingly conventional, straightforward relationships in the story; he loves her for her beauty and wishes to marry her. However, within the context of the story, this paradise-like state is inevitably challenged by the entrance of K, whom Sensei sees as a devil who corrupts him and the situation, bringing, as Sensei remarked earlier to the narrator, guilt into love.