Kim Hyun Joon is an assassin who belongs to a top secret organization known as the "NSS" whereas T.O.P is an assassin who works in another secret organization located in the United States called "IRIS" and was ordered to kill Hyun Joon. Seung Hee is a psychological analyst who plays an important role in bring Hyun Joon into the "NSS." Jin Sa Woo is Hyun Joon's enemy and falls in love with Seung Hee as well.

The story focuses on the conflicts between the young teachers and the problems that high school students have. Jang Yi San is a timid and incompetent classical music teacher who has to teach the only daughter of a highly dangerous mob leader. Eun Byul is the mob leader's daughter and she is also a new student at MyungMoon High but knows nothing whatsoever.