Sung Seo-Yeon (Kang Shin) is an 18 year old high school student. One day, she wins a 5 billion won lottery prize, but she learns that minors are not eligible to receive the prize. To get the money, she approaches 20 year old Hwang Yi-Deun (Jo Joon-Young), who attends the same high school. Sung Seo-Yeon asks Hwang Yi-Deun to become her guardian.
