Kang Hyuk, a middle-school student who earned the nickname "Dokgo" after taking down 30 high-school students, is now a dropout. Two years later, he is a dropout and spends time with his two friends, Choi Jae-Wook and Goo Bon-hwan, who are also dropouts. One day, Hyuk and his friends save Kim Kyu-soon who was getting bullied in an alley. Kyu-soon befriends them and asks for help protecting his sister, Kim Hyun-sun. He explains the threat posed by the Ki Cheon High and Dang Young High school alliances, a pair of delinquents who bully students and extort money to fund their circles. Cho Kang-hoon is the leader of Ki Cheon High, and Kim Sung-kyu leads Dang Young High.
Kim Jong-il, a Ki Cheon High student who does not want to be part of the circle, tries to leave and live a normal life. He is sent with other students to bring Kim Kyu-soon, on orders from Kim Young-ha, the second-in-command of Ki Cheon High. Jong-il demonstrates impressive fighting skills with a pen, and afterward he leaves the circle and promises Kang-hoon never to fight again. Pyo Tae-jin, a wrestler from Dang Young High’s team, crosses paths with Hyuk and his friends in the same alley and befriends them. He brings impressive wrestling prowess and trains for an upcoming tournament. A girl named Cho Ho-rim likes him, and after confessing, they start dating.
After being discharged from the hospital, Kyu-soon and Hyun-sun are taken to Sung-kyu, where Sung-kyu beats them mercilessly. While Jong-il clashes with Dang Young High students by breaking the promise, Hyuk and his friends come to rescue Kyu-soon and Hyun-sun. Later that night, Kyu-soon dies from his injuries. Jong-il is suspended from school and then befriends Hyuk and Tae-jin. When the school refuses to take action on Kyu-soon's death, Hyuk and Jong-il vow to dismantle both Dang Young High and Ki Cheon High. They seek information from Lee Dong-jae and decide to take on the students one by one.
Sung-kyu moves to dissolve the wrestling team as he seeks revenge on Pyo Tae-jin. After the tournament, he lures Ho-rim and assaults her, and Pyo Tae-jin beats him mercilessly until Hyuk intervenes. Hyun-sun urges Sung-kyu to seek forgiveness from both her and Kyu-soon, bringing an end to the Dang Young High circle and the alliance as a whole. Jong-il loses a fight to Myung Jin-hwan, and weeks later Pyo Tae-jin is suspended from Dang Young High while Hyun-sun travels to the States to live with her parents.
Hyuk and Jong-il train together, practicing a fierce matchup that mirrors their broader mission to dismantle the rival factions. Tae-jin later asks to join them in their bid to take down Ki Cheon High. In a decisive moment, Hyuk defeats Kang-hoon, revealing his true identity as Dokgo—a name that signifies a poisonous drum whose sound claims the life of anyone who hears it. Jong-il then defeats Jin-hwan, effectively breaking the delinquent circle of Ki Cheon High. The same night, Hyuk receives a phone call revealing that his older twin brother Kang Hu was beaten nearly to death; months later Hu dies, and Hyuk vows to avenge his death.
