As far as I've heard it was originally planned as a drama with 16 episodes. However, the aired version has 14 episodes.
Story: Ji Seung-yeon wants to become a hip-hop legend and some events seem to make this possible. The only obstacle - she has to dress up as a guy and has to live with the other guys as well. The "M2 Juniors" meet the group M2 while they're being trained by Sunny Entertainment and Seung-yeon happens to meet Kang Woo-hyun who hides a rude and selfish personality behind musical talents and a charming smile.
Cast: The main cast consist of Go Eun-ah as Ji Seung-yeon, Kwak Yong-hwan as Kim Ji-woo, Seung-yeon's best friend, and Park Yoo-hwan as Kang Woo-hyun. While I didn't know any of the actors before Park Yoo-hwan looked somehow familiar. Searching for some info about him I've read that he's Park Yoo-chun's younger brother (and somehow only called Yoo-chun's brother often by English online magazines (AKP *cough*)).
There are also many idols included in the cast or not that typical idols but still singers - there's Song Min-ho more commonly known as Mino of Winner. Back then he was a member of B.o.M, a pretty nice ballad group that disbanded around 2013 if I remember right. Why I'm saying this? Because fellow B.o.M member Maeng Se-chang joined him. Also soloist ALi acted in the drama as vocal teacher Park. And Kevin Kim of ZE:A was another idol included in the cast.
To me this was a 9/10 after all but there could be multiple reasons like simply me being into music dramas. Also the friendship between the members of M2 Junior becomes pretty strong in the end. Though I am not a huge fan of genderbend I really liked this one.
Oh, well, and yeah, I do have a thing for Park Yoo-hwan and Jin Hyuk..
If you like music OR want to see Mino shirtless - watch this. After all it's fun and cute and pretty good.