“Under the Rain” tells the story of Rio Ozawa, a seventeen-year-old high school student who is admired by many for her beauty. However, Rio has never cared about anyone but herself due to her painful past. She sees her friends as mere tools to use for her own benefit, and money is all she cares about. She becomes involved in compensated dating and blackmailing.
One day, Rio’s photos get mixed up at a store with those of another person named Ozawa. This leads her to meet Kouki Ozawa, a thirty-five-year-old university professor, and they fall in love for the first time. Rio is confused by her newfound emotions, transforming from a confident and mature girl to a typical high school student with a fearful heart. She follows Kouki around, asks him to tutor her, and wants to date him.
Although Kouki becomes interested in Rio, there is a reason why he doesn’t pursue a relationship. However, Rio is persistent and starts changing her world to be better for him. Suddenly, Kouki disappears without saying goodbye, leaving Rio devastated. With the help of Kouki’s relatives, her friends discover that he is working at a different school. Rio then finds out that he is dying from brain cancer, which is why he wanted to keep his distance to avoid breaking her heart.
She persuades him to engage in a risky procedure, even if it means he won’t remember her, so they can have more time together. In the end, Kouki survives the surgery, and they meet again, although it’s unclear if he remembers her.