A one-sided love is like a slow-acting poison—you know it's tearing you apart, yet you can't stop coming back for more. For 5 years and 120 days, Jung Dahee has been hopelessly devoted to her childhood friend, Lee Beomseok. Rejection has become such a regular part of her life that she almost expects it, but she's determined to change her fate with one final, heartfelt confession on the day of the CSAT. Everything was supposed to go according to plan. Until she accidentally discovers a shocking secret: the boy she's loved for years is having an affair with none other than her own older sister.