I hope my summary is helpful (my language is Spanish so my written English might not be that good, but I hope you understand it)
I recently read the novel on which this manga is based and also went through the raws. Below is a concise summary of the story, which includes spoilers.
The plot revolves around the couple’s inability to have complete sexual intercourse — a situation that remains unchanged for roughly four years from the beginning of the story. Throughout this time, the protagonist and her husband are never able to have normal sexual relations; their intimacy is limited to oral sex only.
Meanwhile, the protagonist faces numerous difficulties in her professional life as a teacher, which contributes to her declining emotional state. The turning point arrives when she discovers that her husband has been frequently visiting brothels. Although this is clearly infidelity, she ends up partially understanding his reasons, since he also struggled with the inability to consummate their relationship. Still, he begins acting strangely, even going so far as to throw away the meals she prepares.
After a series of personal failures, a suicide attempt, and the shock of discovering her husband’s actions, the protagonist agrees to meet a man she encountered online. Their date results in sexual intercourse, which initially makes her feel guilty, but she eventually realizes that while her husband could not be intimate with her, other men could. From that point on, she begins having sexual encounters with various men in secret, while her husband continues visiting brothels.
This period takes up most of the middle section of the story. Over time, the protagonist reaches her emotional and professional breaking point: she quits her job, and her husband supports her. She then tries to live a more peaceful life. A year later, she is diagnosed with an autoimmune disease, yet she keeps moving forward.
In the final volumes, the story jumps forward about ten years. At age 36, she ends her sexual life with her husband (which had always been limited to oral sex), and it is not clearly stated whether she continues meeting other men or not, though it is implied she probably stopped. At 37, she enters menopause. It is suggested that her husband continues visiting brothels, but this no longer bothers her — partly because she had also been unfaithful, and partly because she came to understand him.
As the years pass, their relationship supposedly grows stronger. She writes a novel, and although her illness progresses, she continues living life to the fullest. The ending provides a bit more information about her husband, but in my opinion, it is not enough. Many of his actions throughout the story lack logic, and he remains a character who is difficult to understand, despite the late attempt to add depth to him.
In conclusion, I do not consider this work particularly noteworthy. Although it was very popular in Japan and sold hundreds of thousands of copies, I found it extremely similar to many other stories centered on infidelity and dysfunctional intimacy. The story also includes several fetishistic elements focused on the protagonist, which I personally found heavy-handed. Many character behaviors lack coherence, their attitudes shift abruptly, and the supposedly philosophical, hopeful ending feels forced. There is no real villain here: both the protagonist and her husband were unfaithful, each in their own way.
I hope this compact summary helps anyone interested in understanding the story without reading it.