Yeong-jae (Sin Joo-ah), Ha-eun (Koo Ji-sung) and Chun-hee (Yoon Chae-i) have been friends for 15 years since high school. Yeon-jae is the youngest of a fine hotel owner and she aims to sleep with 1203 men throughout her life. On the other hand, Chun-hee is an old-school minded high school teacher who has never had sex before in her thirty-something life. Wedding planner Ha-eun was in love with one man for 10 years but recently experienced the pain of losing both her man and work. Then one day, to celebrate their 10th year majority, they have a party of their own in room 1203 of Yeon-jae's hotel. They say the house breaks down if three girls get together.
The lovely and promiscuous textile professor Eun-sook has all the male professors wrapped around her finger. It also helps that she has slept with most of them. Eun-sook is also a member of an environmental awareness group. When the popular comic book artist Suk-gyu joins the group, he arouses the jealousy of Mr. YOO, one of the other member who fears that Suk-gyu will steal Eun-sook from him. It turns out that Eun-sook and Suk-gyu attended the same junior high school where they share a secret and tragic history. Back then, Eun-sook was Suk-gyu's older brother’s girlfriend. But after Eun-sook decides to sleep with Suk-gyu as well, the older brother's best friend dies in an accident in which both of them are involved. As the jealous MR.YOO starts to investigate Eun-sook and Suk-gyu's past, will their secret be kept? Will they get together again?