Kim Yoonhee is a smart, responsible young woman with an independent drive. Since her father passed away, she has worked to support her family, but also managed to do well in her studies. When her younger brother's illness worsens, and their family is in danger of being evicted from their house for lack of money. In her desperation to support him, Yoonhee decides to take the men's only national exam under her younger brother's name and disguises herself as a boy. On the day of the exam, Kim Yoonhee meets Lee Seonjoon and becomes friends. Lee Seonjoon comes from a privileged background and also possesses good looks. He is also arrogant. They both pass the national exam, and she has to share a room together with him at Sungkyunkwan University, while hiding her true nature. There, she becomes friends with the playboy, Goo Yongha, and rebellious Moon Jaeshin.

该剧讲述了金允熙代替病弱的弟弟女扮男装进入成均馆后，与李善俊、文在信、具容河相处的过程中开始的一段朝鲜时代校园热血青春四人帮之间友情与爱情的故事。