Narrator Kae Horii is a 19-year-old college student, active in the mandolin club. One day in August, she returns to her apartment house to find a strange man looking up at where her residence is. Later, the same man shows up at the venerable old stationer's where Kae works part-time. As he tries out a number of their signature fountain pens, she learns that his name is Ryusaku Ishitobi, and that he is an illustrator. She feels the first glow of attraction. Later yet, she finds him looking up at her apartment again, and this time he asks if she might let him inside. They gradually begin to grow closer.
Kae keeps busy between her college classes, mandolin lessons, and part-time job. When she needs a break from her hectic schedule, she reads from a notebook left behind by the previous resident of her apartment?a woman named Ibuki Mano, who was apparently just getting started as a grade-school teacher. The notebook is partly a new teacher's journal, recording her struggles to win the hearts of her first group of pupils, and partly a love diary, detailing her relationship with a certain Takashi: Ibuki had had a secret crush on him in college, but they had gone their separate ways afterwards and had only recently met again. Inspired by Ibuki's journal, Kae decides she wants to become a teacher; and the example of Ibuki's persistent efforts to win over the object of her one-sided love redoubles Kae's determination to deepen her relationship with Ryusaku.
In time, Kae learns that Ibuki died in an accident about six months earlier, on the last day of the school year in March. And that the Takashi referred to in her notebook is in fact Ryusaku, the man Kae now harbors her own unrequited feelings for. (The kanji for Ryu in Ryusaku is commonly read Takashi when it stands alone). In mid-November, at Ryusaku's first solo exhibition of his works, Kae reads the final chapter of Ibuki's notebook to him?expressing the feelings Ibuki had been unable to convey to him while she was still alive.
Obstacles to love, such as the object of one's affection having his heart set on another, or being unable to forget a relationship even after it has ended, have been artfully arranged to produce a poignant tale of unrequited love in its many guises.