Rio Ozawa [Nozomi Sasaki] is a beautiful and manipulative 17-year-old high school student who, along with her best friends Maki [Mitsuki Oishi] and Miho [Araki Nanaki], engages in compensated dating and extortion to earn money. After defending her naïve classmate Tomoko [Hikaru Yamamoto] from the school bully Naoko [Saki Kagami], Rio invites Tomoko to join her clique and soon manipulates her into participating in their compensated dating scheme. It is revealed that Rio and Naoko are secretly lovers, and that Naoko assisted Rio in luring Tomoko into the clique.
One day, a photo developing store accidentally swaps Rio's photographs with those of a man who also bears the Ozawa surname. Rio arranges to meet Kouki Ozawa [Shosuke Tanihara], an introverted 35-year-old history professor, to swap the photos. As they walk together, Kouki faints and is rushed to the hospital. The next day, Rio visits him at the hospital, and they begin to bond. As Rio falls in love with Kouki, she asks him to tutor her, stops doing compensated dating, and ends her romantic relationship with Naoko.
When Rio scores 99 on her history test, Kouki agrees to take her on a date. While in a library one day, Kouki kisses Rio for the first time. Unbeknownst to Rio, Kouki underwent surgery for a brain tumor three years earlier, but the tumor was not completely removed. Kouki's cousin Kaori [Wakana Sakai] tries to convince him to reconsider another surgery, but he does not want to be a burden to others. To avoid hurting Rio, Kouki suddenly distances himself from her, leaving her heartbroken.
Naoko, who has been skipping school, walks into class unexpectedly and tries to stab Tomoko, accusing her of taking Rio away from her, but Rio shields Tomoko. Naoko then runs onto the school rooftop, with Rio in pursuit. Naoko tearfully tells Rio that she had been repeatedly raped by her mother's boyfriend and that her mother blamed her after he left her, so Naoko murdered him. Rio attempts to dissuade Naoko from jumping, but as Tomoko, Maki, and Miho arrive, Naoko jumps to her death after professing her love to Rio.
Distressed by Naoko's suicide, Rio confesses to Tomoko that she was the one who ordered Naoko to bully her, prompting Tomoko to sever ties with Rio. However, when Rio collapses from stress at school, they rekindle their friendship. After Rio laments to her friends that Kouki has moved out of his house without saying goodbye, they convince Kaori to visit Rio at the hospital. Sympathetic, Kaori reveals that Kouki has been transferred to another university and gives Rio his new address.
Rio meets with Kouki and reveals that three years earlier, she became pregnant as a result of rape, so her mother took her to the hospital to have an abortion. She then resorted to prostitution and extortion as a way to avoid getting hurt again, but after meeting Kouki, she realized that seeing a loved one get hurt was much more painful. Kouki acknowledges that Rio is no longer the same person. Shortly after Rio boards a bus, Kouki faints.
Rio accompanies Kouki to the hospital and learns of his brain tumor. When she visits him the next day, he tells her that he never expected to fall in love with a girl like her knowing much time he had left to live, but since meeting her, he began wishing for more time with her. Rio urges Kouki to undergo another surgery, even at the risk of losing his memories. He reluctantly agrees, and shortly before the surgery, he finally tells Rio that he loves her.
Some time later, Rio graduates from high school. On a rainy day, she returns to the hospital to find Kouki, who survived the surgery. Though he does not seem to remember her, he agrees to walk her to the train station under his umbrella. After Kouki walks away, a tearful Rio stands in the rain. Shortly afterwards, Kouki returns and covers Rio under his umbrella, and she joyfully embraces him.
