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 (Park Jin-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. 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...