Doo-sik (JUNG Jun-ho) the number two of a leading gang the D-Dogs is full of ambition and expectation for his once in a life time opportunity to be back to school not as a student not a guard but as a student teacher! It's really something for him 'cause he already failed to complete his high school course years ago. Now it's kind of a second chance for him. He just interprets this teaching practice as something like a social adaptation course for the prisoners who are going to be freed and back to society soon. But has he got what it takes as a teacher? Even on the first day he is just out of the way and eccentric. He enters school through the dog hole. And it doesn't take long to sum up that his behavior is just way too much for the students to accept and respect in a teacher. And as the fate would have it his own boss the leader of the D-Dogs is now right in front of him as one of the students and two of his loyal men are out of reach every time he's in trouble! Doo-sik just dashes this way and rushes that way and all the incidents are leading Doo-sik to a place no one expects.