This drama tells the story about a 20+ year old rookie detective, Eun Dae-gu at a police station in Gangnam.
Meanwhile, Seo Pan-seok is the team leader of the violent crimes unit. Eo Soo-sun is the sole female in the new class of officers.
For the first time ever, the police have four new recruits as a team. The fateful first meeting at Gangnam police station brings a busy first day for the team. On the other hand, Dae Gu can't forget the incident that happened 11 years ago that also involved Pan Suk.
The clumsy new team brings trouble to the Gangnam police station on their first day. When they are given a chance to prove themselves while undercover, things don't go as planned.
Team leader Pan Seok is given 3 months to train the rookies yet he doesn't think they can actually be detectives. He lets each of the partners have their own case. Dae Gu is able to figure out the clues with his photographic memory but because of his rashness, it may be too late.
Dae Gu and Soo Sun are dealing with the aftermath of their first case. As they are trying to overcome their mistakes, an unexpected event happens. The team is forced to work together.
A seven-year-old boy is killed in a hit-and-run accident. The P4 search for clues to find the culprit. Team Leader Seo is reminded of a tragedy from his past.
Team Leader Seo resorts to violence when problems arise during the team's recent case. The P4 continue to work together to catch the real criminal. Meanwhile, a mysterious man shows up looking for Dae Gu.
After a surprise kiss between Dae Gu and Soo Sun, they become awkward around each other. A witness is scared to identify a criminal, which causes an argument between Dae Gu and Pan Seok. Pan Seok decides to look into Dae Gu's past.
Dae Gu has to choose between Pan Seok's meeting with the mysterious man or catching a criminal. Pan Seok begins to figure out Dae Gu's identity. The P4's new case involves two brothers and the death of their father.
The case of the dead father is solved. While working on their next assignment, Soo Sun learns Dae Gu's real identity. Using DNA testing, Pan Seok also confirms his suspicions.
Finding out Dae Gu's identity brings back memories from 11 years ago. The relationship between Pan Seok and the mysterious man with the boots is revealed. The team tries to find a missing person.
The misunderstanding between Pan Seok and Dae Gu starts to clear up. The rest of the team is let in on the incident from 11 years ago. Soo Sun is determined to help Dae Gu by finding the culprit.
Now that the man from Dae Gu's past has been arrested, the team tries to find his accomplice. A problem arises with Soo Sun's mother.
While investigating what happened to Soo Sun's mom, Pan Suk's team might be opening another can of worms.
The team has picked a fight that will be very difficult to win as they are going after a politician's daughter.
An assignment may have brought out real feelings between partners.
A necklace Soo Sun has had all this time is the key to what really happened to Dae Gu's mom.
Their evidence may be gone but the case will continue no matter how much Yoo Ae Yun fights them.
Dae Gu and his team are close to proving Yoo Ae Yun was involved in his mom's murder but there are still things he doesn't know.
An attack on the Chief gives everyone even more reason to get to the bottom of their case.
They now know who is behind everything so Dae Gu, Pan Suk and their team just need to prove everything.