In the neon glow of Gangnam, a girl flees into the street to escape her pursuers; detective Kang Dong-woo, broker Yoon Gil-ho and prosecutor Min Seo-jin are each pulled into yet another missing persons' case in Gangnam.

When a body is found at Gangnam Station, a Special Investigations Unit is established; detective Kang Dongwoo returns to Gangnam and teams up with Kim Jangho to solve the mystery; Yoon Gilho encounters K-pop star Noh Joonseo.

Dongwoo has an unexpected encounter when he investigates in Gangnam using the crucial piece of evidence left by Yoon Gilho; 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.

Jaehee heads over to meet Choi Hakgu to save Yeseo; Dongwoo and Gilho try to rescue Jaehee.

Seojin climbs the ranks with the help of Hakgu; Dongwoo becomes a chauffer and hunts for Yeseo.