The story follows Saito Yudai, a 27-year-old office worker who feels his life lacks purpose, characterized by his "dead fish eyes" and cynical personality. His life is a monotonous cycle of work and solitude, exacerbated by a past humiliation in high school. During his senior year, he was the target of a prank by a popular girl, Nishino Mio, who confessed to him as a dare. This led to him being publicly ridiculed when a video of their staged dates was uploaded online. This event deeply affected Yudai, solidifying his cynical outlook.
The narrative shifts to the present, where Yudai unexpectedly encounters Nishino Mio again. She has moved into his apartment building as his neighbor. Despite their shared past, Mio appears to have no recollection of the prank, treating Yudai with friendly familiarity. She is depicted as a beautiful, successful, and outgoing woman, a stark contrast to Yudai's introverted nature. Their interactions are marked by Yudai's internal struggle to reconcile his past trauma with Mio's current, seemingly innocent demeanor. He constantly reminds himself that she is just a neighbor and that he must not fall for any perceived advances, fearing a repeat of his past humiliation.
Their neighborly interactions become more frequent, with Mio often initiating conversations and suggesting activities. Yudai finds himself reluctantly agreeing to these invitations, such as walking to the station together or going out for meals. He rationalizes these encounters as her simply being friendly and him being convenient due to his lack of plans. He observes her social ease and popularity, contrasting it with his own isolation.
A significant turning point occurs when Yudai receives a high school reunion invitation. This triggers a flood of memories about his past with Mio, reinforcing his belief that she is skilled at deception and that her current friendliness might be another form of manipulation. He resolves not to attend the reunion, feeling their lives are too different.
Meanwhile, Yudai's subordinate, Sakura, who recently transferred to his department, begins to show him an unusual amount of attention. Initially perceived as a strong-willed and beautiful individual, Sakura starts bringing Yudai coffee daily and engaging in playful banter, often touching him. Yudai is bewildered by this change, as she was previously cold towards him. He suspects she might be trying to trap him or is simply being overly friendly, but he also notes her genuine kindness and support, which reminds him of his own past experiences with Mio.
The story also delves into Yudai's internal world through dreams and reflections. He dreams of Mio confessing her love, which further unsettles him and highlights his deep-seated anxieties about his past. He tries to cope by exercising and focusing on the present, reinforcing the idea that he and Mio are merely neighbors with no shared history.
The chapters culminate in Yudai agreeing to have hot pot with Mio at her place, a decision he makes despite his reservations. He also experiences a haircut that turns out shorter than expected, but surprisingly, he finds he doesn't entirely dislike the new look. At work, Sakura's continued attention and compliments, along with her offering him coffee, leave him feeling confused and slightly uneasy, questioning her motives and his own perceptions. The narrative consistently portrays Yudai's internal conflict between his past trauma and his present interactions with Mio and Sakura, leaving the reader to wonder about the true nature of these relationships and Yudai's emotional journey.
