Teacher, what is that iron wall?

Yeo-ju and Min Yoon-gi, who are bad friends, start attending private tutoring one day. While looking for tutoring, they see a private tutoring company that offers expensive tutoring. They go to see them and meet Kim Tae-hyung. They are attracted to his good looks and while tutoring him, Min Yoon-gi and Kim Tae-hyung gradually develop feelings for Yeo-ju, and soon a love triangle is formed. Who will fall in love with Yeo-ju? ..