Han Seung-wan leaves for Singapore to surprise his girlfried on her birthday. Jung Se-jin is on the same flight - she has worked hard to save money for her trip to Singapore to say farewell to her deceased "first love" who died saving a tourist in Singapore. Due to a series of incidences at the airport, they mistakenly take each other's passports. They meet again at the police station when Seung-wan is arrested for attempting to break into his girlfriend's apartment wearing a spiderman suit! Seung-wan and Se-jin end up spending the day together comforting each other - Se-jin sights his girlfriend with another and Seung-wan when she thinks of her lost love. They inadvertently spend the night together.
Back in Korea, Se-jin discovers she is pregnant. She tries to tell Seung-wan, but he is too engrossed with getting back with his girlfriend that she changes her mind. She leaves to have the baby alone.
A series of events leads to Seung-wan discovering that he has fathered a child with Se-jin and that Se-jin has "died". Seung-wan's life is thrown into array as he is now forced to face fatherhood and the anger of his girlfriend - Lee Chae-young - who vows never to see him again. Se-jin reappears and the couple is forced to get married by their respective families. The rest of the story revolves around the couple journey in learning to take responsibility, and learning to love each other.
Lee Ji Hoon as Min Do-hyun
Han Eun Jung as Lee Chae-young
Joo Hyun as Han Bum-soo
Sun Woo Yong Nyeo as Yoon Tae-hee
Kim Hye Ok as Pyo Jae-kyung
Choi Joon Yong as Han Seung-pil
Yoon Hyun Sook as Baek Hyun-joo
Kim Hyo Jin as Jung Il-jin
Kim Seung Min as So Chang-myung
Jung Da Bin as Han Shin-bi