Seventeen-year-old Hotaru Morikawa is an ordinary high school student who develops an intense infatuation with her biology teacher, Sakuya Kagetsu. Driven by the innocent desire to get closer to him, she attempts to bridge the distance between student and teacher, unaware that the object of her affection harbors a hidden and deeply traumatic past. As Hotaru pursues her feelings, she is gradually drawn into a dark whirlpool of crimes that surround Kagetsu, and the story pivots from a forbidden school romance into a tense psychological mystery.

The manga's setting is a contemporary Japanese high school, where Hotaru's energetic and sometimes oblivious personality contrasts with Kagetsu's calm and secretive demeanor. The central conflict unfolds as Hotaru discovers that Kagetsu is married to a woman named Natsumi, yet there are unexplained gaps in his memory and behavior. She learns that Kagetsu has been suffering from memory disturbances, and his wife seems to have vanished under suspicious circumstances. As Hotaru digs deeper, she begins to question what she knows about love, trust, and the nature of the "crime" hinted at in the title. Key narrative arcs include the early phase of Hotaru's one-sided pursuit, the revelation of Kagetsu's traumatic backstory and the existence of his wife, the investigation into Natsumi's disappearance involving a mutual friend named Yuka Hiiragi, and the gradual shift from a lighthearted crush to a suspense-driven unraveling of secrets and lies. The story culminates in a climax where the characters must confront the consequences of their choices and the truth behind the title's dual meaning of love and transgression.