Four rookie detectives meet in the streets of Gangnam, and they recall how their journey began 11 years ago, in a high school far from the center of Seoul; as students, they witnessed crime after crime.

The rookie detectives take on their first case; Dae Koo recognizes Pan Seok and holds a grudge, but Pan Seok doesn't remember him; though Dae Koo hasn't revealed his identity to him, he's makes sure to learn more about Pan Seok.

After the rookies' incident, Pan Seok calls them the worst the station has ever seen and wonders how Dae Koo, Soo Sun, Tae Il and Kook graduated from the police academy; he grants them a second chance of three months to teach them all a lesson.

Dae Koo and Soo Sun screw up on a case, and they get in big trouble after everyone finds out; Dae Koo and Pan Seok get into a fight, but the case causes another scandal to the Gangnam Police Station.

The four celebrate their victory after knocking down the hostage-taker; Soo Sun thinks of living with the three boys, but Dae Koo isn't very happy about its idea; Dae Koo follows Pan Seok closely, still hoping to find an answer.

Pan Seok gets put behind bars after assaulting the prosecutor; Dae Koo finds more leads and dead ends to his personal case; Soo Sun sets out to resolve the case and sees a familiar face in Dae Koo wearing his old pair of glasses.