The protagonist, Haruna Iba, is a university student in Kitakyushu. One evening, she visits the general hospital where her brother works. Her brother, Masato, a surgeon, had just finished operating on an emergency patient when he said something strange to her: "The locket pendant the patient had contains a photo of me as a child. Why is that?" Soon after, they learn an unexpected truth from their parents—the woman who had been brought in, Yumiko Sakurai, is Masato's biological mother. As their father reveals more about the past, they are further shocked by the revelations.

Minami Yoshimura, a high school senior living in Itoshima City, meets 26-year-old hairstylist Ren Hatae at a small salon by the sea. Instantly smitten by Ren’s striking looks, Minami launches a relentless romantic offensive. However, Ren brushes her off with the calm composure of an adult, telling her, "If you still feel the same way when you turn 20, then we’ll see." Undeterred, Minami’s unwavering devotion eventually pays off, and the two are finally united. But their happy days come to an abrupt end when Minami leaves to study abroad in South Korea. Out of nowhere, Ren breaks up with her without giving a single reason. Years later, Minami is confronted with a truth that is far too harsh, a "cruel twist of fate" that makes one want to resent God himself. Burdened by a secret they can tell no one, the two ultimately choose to embark on a "thorny love."