Based on Yayaoi Ogawa's Japanese manga "Kimi wa Pet" serialized from 2000 to 2005, the story is about Eun-Yi works as an editor for a fashion magazine. She keeps failing in relationships with men, but doesn't get along with her colleagues. In-Ho was a promising ballet dancer. Two years ago he lifted a female dancer, but dropped her on accident. Because of this accident the woman was unable to dance any further and, now, In-Ho won't dance with women anymore. In-Ho now hopes to become a musical choreographer. Unfortunately for In-Ho, he doesn't have a place to stay. He goes to a bar where his friend Eun-Soo works. Later that night, Eun-Soo takes In-Ho over to his older sister's house.
When Eun-Yi arrives at her home, she walks into the bathroom and sees someone brushing her teeth. As she walks by she pats him on his rear end, assuming the guy is her younger brother Eun-Soo. When the guy turns around she's shocked that it is a complete stranger. Eun-Soo then asks his older sister Eun-Yi if In-Ho can stay at her place temporarily, telling her that he's really good with household chores. Eun-Yi disapproves of the idea, but that night Eun-Soo accepts money from In-Ho to stay at her sister's place.
The next evening, on a rainy night, Eun-Yi walks home and finds a large blue box on front of her home.
When she looks inside of the box she's shocked again to see the guy the from the other day, In-Ho, sitting in the box. Ji-Eun brings sopping wet In-Ho into her apartment, while she tries to contact her younger brother. Her younger brother doesn't answer her calls. Oddly enough, In-Ho reminds Eun-Yi of her former pet dog Momo. Eun-Yi then comes up with an arrangement. She will let In-Ho stay at her apartment if he will become her pet dog named Momo.
As they become more familiar and comfortable with each other, Eun-Yi's first love Cha Woo-Sung comes to her office looking for her...
Kim Ha-Neul as Ji Eun-Yi
Jang Keun-Suk as Kang In-Ho
Ryu Tae-Joon as Cha Woo-Sung
Jung Yoo-Mi as Lee Young-Eun
Choi Jong-Hoon as Ji Eun-Soo