《原来是美男啊》讲述了年轻偶像组合成员的成长过程以及围绕他们发生的爱情和纠葛。讲述了淳朴少女偶然成为男子偶像组合的一员后所经历的各种故事。通过张根硕饰演的男主演的成长历程，向人们展示了少年偶像们普通人的一面和他们在华丽的舞台背后经历的鲜为人知的痛苦。而朴信惠则是将一人饰演双胞胎兄妹，与张根硕上演浪漫爱情戏

At the height of popularity, the 3-membered boy band, A.N.JELL faces a looming stagnation. To overcome it, the promotion company holds a sensational audition for the 4th member. Mi-nam is selected to be the one and joins the recording of the new album. But due to an injury, he needs to be hospitalized for 3 months. Unable to bear the thought of letting her brother’s dream go down the drain, Min-nyu dresses up as a boy to take her brother’s place in the group. Despite her misgivings, the other members don’t seem to notice. Mi-nyu is very conscious of Shin-woo, whom she used to idolize. Just before their new song is released, her secret is discovered. The company decides to take her out and record the song without her. But the song has already spread throughout the internet. The new song became so popular that it was impossible to take Mi-nyu out of the group. They decide to let her stay just for this album.

In order to keep the interest of media and fans away from her, they fabricate a scandal for the group's leader Tae-kyung, with popular actress Yoo He-yi. After a while, He-yi finds out that ‘Mi-nam’ is, in fact, a girl and by that time, she came to like Tae-kyung very much. The presence of a girl by his side bothers He-yi, so she begins to harass Mi-nyu. Tae-kyung and Mi-nyu become friends through quarreling, and he helps her to get close to Shin-woo. But watching Mi-nyu and Shin-woo’s close relationship, Tae-kyung finds himself strangely bothered by the two..