Baek Do Ha and Baek Do Yeong are twin brothers, but have lived apart since their parents' divorce when they were young. Younger brother Baek Do Ha lives in the United States, and older brother Baek Do Yeong lives in South Korea. Baek Do Ha comes to South Korea and stays in a duplex house with his father and brother, Baek Do Yeong, every summer for 21 days.
During this time, he spends time with Baek Do Yeong and his next-door neighbor Song Ha Gyeong. Song Ha Gyeong's father is also good friends with the twins' father. Song Ha Gyeong grows up bickering with summer guest Baek Do Ha, but grows to have feelings for him. She is unable to express her feelings to him, who only visits in the summer.
Now, as adults, Baek Do Ha works as a talented architect, and Song Ha Gyeong works as an architectural public official. Because of an incident of 2 years ago, Song Ha Gyeong and the twin brothers face irreversible changes in their lives.
(Source: AsianWiki)
