To maintain their club and get into a music competition, Gun and his band members must all earn passing grades on upcoming exams. Otherwise, the principal will not sign off on the competition paperwork.
Tinn agrees to tutor the band members, specifically Gun. He imagines a scenario where his explanations are immediately grasped and implemented by the appreciative Gun.
Alas, if only tutoring was that easy! 
Tinn finds, first of all, that mathematical concepts don't come easy to Gun; next, Tinn isn't as patient a guy as he thought he was; finally, although Gun is fully willing to commit himself to a full night of exhausting singing performances, he is far less willing to focus on his academic workbooks.
To be "nice," Tinn has to be "mean" and insist that Gun not stop working the moment he gets bored.
In terms of romance, it is a great example of reality meaning more than sentiment. The relationship isn't built on cutesy moments (okay, not only cutesy moments) but on the young men dealing with each other as they are.
