A student, Go Nam Soon (Lee Jong Suk), studies at a Seungri High School, where he is always bullied by a gang led by Oh Jung Ho (Kwak Jung Wook). At the beginning of the school day, his class 2 gets a new homeroom teacher called Jung In Jae (Jang Na Ra), who finds it hard to control the class. Later, a new teacher named Kang Sae Chan (Choi Daniel) arrives at school. He is appointed as co-homeroom teacher to class 2. The story introduces the remaining students and their blossoming friendships. A new transfer student named Park Heung Soo (Kim Woo Bin) joins the school. He knows Go Nam Soon and is holding a grudge against him. As the story rolls on, we get to know their past relationship and what has caused the rift between them. Meanwhile, Teacher Jung and Teacher Kang help the students overcome their problems. The story ends with everyone moving up to Class 3.