Bo-eun (Moon Geun-young) is an ordinary high school girl who worries about SAT’s and has a crush on her school's baseball team ace, named Jung-woo. One day, Bo-eun’s grandfather orders her to marry Sang-min (Kim Rae-won). When they were very young, Bo-eun’s and Sang-min’s grandfathers had promised that they would become in-laws. Sang-min and Bo-eun have known each other since childhood but Sang-min, now a womanizing university student, always makes fun of Bo-eun and treats her like a baby. Apparently, neither of them are happy: Sang-min doesn’t want to give up his freedom to hang around with gorgeous girls, and Bo-eun’s heart is broken because she loves Jung-woo.
Despite their opposition, they are forced to marry because of the strong influence of Bo-eun’s grandfather. Bo-eun’s undercover married life begins: She pretends that she doesn’t have a husband and starts dating Jung-woo. Bo-eun believes that she can manage both men and live a double life. Everything goes smoothly until Sang-min visits Bo-eun’s school as a student teacher, and finds out about Jung-woo.
Will Bo-eun figure out who is the love of her life? Is Sang-min going to stop chasing girls and settle down to married life?