The P4—also known as Eun Dae Gu, Uh Soo Sun, Park Tae Il, and Ji Gook—are the newest members of the Gangnam Police Station’s violent crime section. These new hires joined the crime squad for a variety of inappropriate reasons, none of which show enthusiasm or excitement for their new positions. Seo Pan Suk, their unpredictable and irrational leader, is only interested in getting rid of the inept new recruits.
Kim Ji-yong (Lee Seung-gi), who witnessed his mother’s murder when he was a young child, trained as a detective to look into his mother’s death. Later, to avoid being discovered by his mother’s killer, he changed his name to Eun Dae-gu. On the way, he runs across Eo Soo-sun (Go Ara), a former classmate with whom he eventually strikes up a romantic relationship. His suspicion that Seo Pan-seok (Cha Seung-won), his team leader, is related to the murderer of his mother leads him to spy on him at the same time. He learns about his past and identity as he pieces together the facts of the incident from 11 years ago, and he forges strong bonds with others around him.