The story starts in a small town in Gunma Prefecture and follows Takao Kasuga, a middle school bookworm whose favorite book is Charles Baudelaire‘s Les Fleurs du mal. One day after school, he impulsively steals the gym clothes of Nanako Saeki, the classmate he idolizes. However, a girl named Sawa Nakamura sees him and blackmails Kasuga into a “contract”. At the same time, Kasuga grows closer to Saeki and manages to become her boyfriend. As Kasuga spends more time with both girls, he finds the guilt of his theft weighing down on him. He attempts to confess by vandalizing his classroom with Nakamura, but Saeki refuses to break up with him. When his mother finds out he was responsible for the vandalism, he runs from home and attempts to bike with Nakamura past the mountain adjacent to the town. Saeki catches up to the pair as they rest and tries to make Kasuga come back. Unable to choose between them, he estranges both girls and the police collect them.
A month later, Kasuga breaks up with Saeki and resolves to help Nakamura. When she runs from him, Kasuga goes to her house and reads in her diary about her disappointment in not being able to reach “the other side”. He steals the panties of all the girls in his class except Saeki’s and uses them to decorate a makeshift hideout, winning Nakamura’s approval. At the start of summer vacation, they create a plan to nail the panties to a piece of plywood to display at the upcoming festival. Saeki discovers the plan and lures Kasuga to the hideout, trying to seduce him and make him stay in their town. When he chooses Nakamura over her, she rapes him but Kasuga resists, causing her to burn the hideout down. Saeki later turns herself in to the police for setting the fire, prompting her best friend Ai Kinoshita to tell their school about Kasuga’s crimes. The school doesn’t involve the police and his parents decide to move over the vacation. The day before the festival, Nakamura breaks into Kasuga’s house, attacking his father, and the two escape. At the festival, they don disguises and climb to the top of a float while wielding a knife. They curse their town and pour kerosene on themselves, but before they can use a lighter Nakamura pushes Kasuga over the float and she gets tackled by her father.
A few years later, Kasuga is going to high school in Ōmiya-ku, Saitama and still can’t forget Nakamura. Kasuga finds his classmate Aya Tokiwa looking at Les Fleurs du mal in a used bookstore and she starts lending him novels, rekindling his love of literature. Kasuga discovers that she is working on a novel and is bought to tears upon reading the manuscript because he can identify with the protagonist. Kasuga encounters Saeki and when they meet up for lunch, she accuses him of using Tokiwa as a stand-in for Nakamura. Kasuga visits Tokiwa at her workplace and asks her out, saying that he will save her, and she accepts. Kasuga visits his hometown for his dying grandfather and ends up meeting Kinoshita, who regrets being left behind by Saeki and tells Kasuga where Nakamura moved. Tokiwa finishes her novel, but Kasuga tells her about his past and his desire to meet Nakamura again. They take the train to Tokawa Station in Chōshi, Chiba and find her eatery, where her mother warns them that Nakamura is “peaceful now”, but Tokiwa insists on speaking to her. They talk at a nearby beach and Kasuga questions her, but doesn’t get satisfactory answers. The three roughhouse and Kasuga tells Nakamura that he is happy that she didn’t disappear. Later, Kasuga is in college and still dating Tokiwa, who is working on another novel. Kasuga dreams about his future with her and about his friends reuniting with loved ones. When he wakes up, Kasuga starts writing in his empty book, starting with the story of him meeting Nakamura.