It’s love at first sight when Seo In-ha (Jang Keun-suk) and Kim Yoon-hee (Im Yoon-ah) meet as shy university students in the seventies. It takes time for the hesitant pair to finally voice their feelings, but circumstances beyond their control send them down different paths. In 2012, In-ha (Jung Jin-young) is now unhappily married to Baek Hye-jung, one of Yoon-hee’s former best friends. He has never gotten over his first love, and when he runs into Yoon-hee (Lee Mi-sook) after so many years, the two reconcile and look forward to making up for lost time.
In-ha’s son Seo Joon (Jang Keun-suk) is a photographer, and he bumps into Yoon-hee’s daughter Jung Ha-na (Im Yoon-ah) by chance. Unlike her mother, Ha-na is a cheerful and vivacious girl, and though they initially find each other troublesome, even as they argue, they can’t help but be drawn to each other. Gradually their feelings develop, and Joon and Ha-na fall in love.
Unaware that their children are dating each other, Seo In-ha (Jung Jin-young) and Kim Yoon-hee (Lee Mi-sook) announce that they are getting married, which will make Joon and Ha-na step-siblings. As both couples deal with the shock, Joon and Ha-na struggle with sacrificing their love for their parents’ long-aborted happiness