Hong Sul's life is filled with work and study, and her diligent attitude is often taken advantage of by others. Yoo Jung was similar in the past, but has developed an exacting system of give-and-take to protect himself. Behind his solicitous smile lies a cold mind that demands payment for any mistakes it notes. When he wants to start dating Sul, she finds herself caught in a tangle of problems as the consequences of his past and present actions come to light.