Lee Yeoreum, a college student who carries the pain of her first love. One day, her first love, Ji Sooyoung, suddenly confesses his feelings to her. Yeoreum finds herself swayed by Sooyoung's fervent pursuit, which goes as far as him breaking off his engagement. After much contemplation, Yeoreum decides to date him for exactly one year in a relationship where she'll be the toxic one, in order to hurt him! However, Sooyoung's confession held a secret...