Han Seung-wan (Kim Jae Won) leaves for Singapore to surprise his girlfried on her birthday, and Jung Se-jin (Eugene) is on the same flight to say farewell to her deceased "first love" who died in Singapore. When literally bumping into each other at the airport, they accidentally take each other's passports. They meet again and Seung-wan and Se-jin end up spending the day together. Se-jin knows about his girlfriend, who is two timing Seung-Wan with Min Do-Hyuk (Lee Ji Hoon). Partially to help Seung-Wan avoid from discovering the truth, Se-Jin spends the rest of her time with him. They both get drunk out of sadness of their loves, and end up sleeping together. Back in Korea, Se-jin discovers she is pregnant. She leaves to have the baby alone. A series of events happens for about a year, and Se-Jin's clueless sister ends up spilling the secret to Seung-Wan at this engagement party, showing up with Han Shin-Bi (Jung Da Bin), his daughter, without Se-Jin. Seung-wan's life is forced to face fatherhood and the anger of his two-timing girlfriend - Lee Chae-young (Han Eun Jung). Se-jin reappears as they are forced to marry by their respective families. Complicated love takes place, as Do-Hyuk starts to fall for Se-Jin, and so does Seung-Wan, though Se-Jin is more focused on her loving daughter, as something surprising happens to her. The rest of the story revolves around the family's journey in learning to take responsibility, and learning to love each other.