The story follows Asano Yuichi, a high school student who feels his life has been a monotonous, gray existence for the past year. He struggles academically, lacks athletic ability, and is socially awkward, making it difficult to form friendships. His life begins to change with the unusual attention he receives from his Japanese literature teacher, Morihara Yui.
Morihara-sensei, known for her cool demeanor, intellectual beauty, and popularity among students, starts to exhibit increasingly close behavior towards Asano. This begins on class assignment day when she asks for his help with a high poster, leading to their first physical contact. Later, while putting away supplies, she guides his hand, and their shoulders brush, creating a palpable warmth. She then thanks him and mistakenly calls him "Suenaga," a classmate's name, before bidding him farewell. This marked the beginning of Asano's days becoming "slightly, faintly colored."
The unusual interactions continue. After Asano scores zero on a pop quiz due to falling asleep, Morihara-sensei calls him for a remedial session. During this session, she sits unusually close to him, their shoulders touching. She questions if his falling asleep was due to her lesson being boring, but Asano denies it, revealing he had been studying late. Morihara-sensei acknowledges his hard work and advises him not to overexert himself.
One rainy afternoon, Asano is caught without an umbrella and seeks shelter in a park gazebo. Morihara-sensei appears, dressed casually, and offers him her umbrella. She correctly guesses his reason for being out and insists on walking him home, their shoulders touching under her small umbrella. She reveals she lives directly across from his house, a fact that surprises Asano.
The next day, Asano wakes up feeling unwell with a fever. Morihara-sensei visits him at school, expressing concern and touching his forehead. She confirms he has a fever and escorts him to the infirmary. While he rests, she assures him she'll be back. Later, he experiences a dream-like state with soft voices and warmth.
Meanwhile, Okuno Manami, a classmate who has developed a crush on Asano, witnesses him and Morihara-sensei kissing in the hallway. Confused and shocked, she confronts Asano, who, trying to cover up the situation, claims their closeness is a result of "flow" and "a lesson." His explanation only deepens Okuno's misunderstanding, leading her to believe they are dating. Morihara-sensei observes this interaction from the hallway, her expression unreadable.
Asano recovers and returns to school, where Morihara-sensei's physical closeness intensifies, making him question why no one else notices. Okuno approaches him again, offering her notes and asking about his relationship with Morihara-sensei. Asano's attempts to explain away their interactions as a teacher-student dynamic fail, and Okuno flees in distress. Morihara-sensei again observes this from the hallway.
Morihara-sensei is absent from school the following day due to illness. Asano feels guilty, believing he may have infected her. He goes to her house to check on her and finds her with a fever. He insists on helping her, and she invites him inside. While tending to her, he discovers her hidden collection of suggestive novels, which she found under his bed. To equalize the situation, she removes her glasses, revealing a more vulnerable side.
Later, Asano visits Morihara-sensei's house again. They share pudding, and she initiates an indirect kiss by feeding him from a spoon. She then has him taste her pudding, leading to another indirect kiss. She explains her actions as a way to "replenish" the missed lesson time, though her words are ambiguous.
The next day, Morihara-sensei recommends several books to Asano in the library, including romance novels and books about mysteries and forbidden relationships, hinting at her own feelings. She also jokes about him writing a "review" of the books.
The chapters conclude with Asano reflecting on Morihara-sensei's increasingly close and enigmatic behavior, questioning her motivations and his own feelings.