In the neon glow of Gangnam, a call girl flees into the street to escape her pursuers. Detective Kang Dongwoo, broker Yoon Gilho, and prosecutor Min Seojin are each pulled into yet another missing persons' case in Gangnam.

When a body is found at Gangnam Station, a Special Investigations Unit is formed. Detective Kang returns to Gangnam and tries to solve the case. Gilho thinks K-pop star Noh Joonseo is behind Junghwa's case, but soon falls into a dangerous trap.

As Dongwoo investigates in Gangnam with the crucial piece of evidence left by Yoon Gilho, he has an unexpected encounter. When the investigation takes a dark turn, Seojin asks for Dongwoo's help. Gilho receives unsettling news while chasing Jaehee.

After being rescued by Seojin, Gilho tells Dongwoo and Seojin that the Colosseum holds the key to the Gangnam missing persons' case. Jaehee rushes to the Colosseum to save her abducted friend, and Dongwoo, Gilho, and Seojin are right behind her.

Tak Juil abandons Seojin after she loses the prime suspect. Jaehee heads over to meet Choi Hakgu in order to rescue Yeseo. As tensions rise, Gilho and Dongwoo return to Gangnam to bring Jaehee back, but she is determined to follow her own plan.

Seojin climbs through the ranks and is closer to achieving Hakgu's goals. Dongwoo turns in his badge and takes a job as a chauffeur to search for Yeseo. As he tracks the drug distribution network, he uncovers the darker, grittier side of Gangnam.