Season 1

Si-eun is a model student who is always at the top of his school. Because he only cares about his studies, he has no friends at school. Young-bin, a boy in Si-eun's class, is annoyed by Si-eun's cynical attitude. On a mock test day, Young-bin decides to use Bum-seok, a new student, to mess with Si-eun's test.

Young-bin wants revenge on Si-eun for putting him in the hospital by hiring a couple of thugs he knows. Meanwhile, Si-eun gets closer to Soo-ho and becomes friends. Later, Soo-ho comes to Si-eun's rescue when he gets confronted by Young-bin and his thugs.

When Gil-su sees his boys beaten up to a pulp, he gets furious and asks who did it. Young-bin lures Si-eun, Bum-seok, and Soo-ho and takes them to Gil-su. Gil-su threatens them to bring him 15 million won in three days. Si-eun asks Young for advice and information on Gil-su.

Si-eun sees Soo-ho's blood on the floor and calls Young-yi for help. Gil-su tells Seok-dae to come, so Young-yi follows him with her GPS tracking app on. Meanwhile, Soo-ho and Bum-seok are tied up by Gil-su's gang.

While hanging out in a karaoke room with his friends, Bum-seok runs into the bullies from his previous school. Soo-ho makes them apologize, but Bum-seok isn't happy with Soo-ho ordering him around. Si-eun becomes more worried about Bum-seok who seems to be getting more unstable.

Bum-seok befriends with Jeong-chan and Tae-hoon, he reeks of alcohol every day. Si-eun thinks Bum-seok is being used by Jeong-chan and Tae-hoon. He also thinks Bum-seok is behaving that way because he is lonely. Si-eun wants to help Bum-seok, but Soo-ho thinks he's done enough. Meanwhile, Bum-seok pays Woo-young to beat Soo-ho on his birthday.

Si-eun gets beaten up by the guys Bum-seok paid to beat up Soo-ho. Si-eun gets seriously injured, but he doesn't tell Soo-ho about it to protect Bum-seok. Soo-ho finds out later and decides to confront Bum-seok. Bum-seok pays Woo-young again to beat up Soo-ho.

Si-eun and Young-yi get anxious as they can't get hold of Soo-ho for too long. After Si-eun finishes his exams, he hears the shocking news. Meanwhile, Bum-seok's dad tries to make Bum-seok go study abroad.