A body at Gangnam Station prompts a Special Investigations Unit. Detective Kang reunites with Kim Jang-ho as Gil-ho suspects K-pop star Noh Joon-seo.

Dong-woo investigates with evidence from Gil-ho and meets an unexpected figure. Seo-jin seeks help as the case deepens while Gil-ho gets unsettling news.

Rescued, Gil-ho reveals the Colosseum is the key to the case. Jae-hee rushes to save her friend as Dong-woo and Seo-jin follow with their own motives.