Ryung and Ji Seok work together to save Jung Hoon and catch the kidnapper. With their great partnership, they discover the identity of the person who helps the kidnapper hide from the police. Meanwhile, the Subway Ghost marks his next target and Ryung activates her subway search skills to track him down.

Ma Ri leaves Ryung after saying harsh words that keep replaying in Ryung's mind. Still, Ryung continues her search for the Subway Ghost. But this time, she comes across someone willing to help. Meanwhile, an autistic boy goes missing from the bakery in Wangsoori Station and the Grasshoppers are on a pickpocketing spree once again. Ji Seok and Ryung have different priorities in which case to pursue.

Subway Police and the Metropolitan Detective Division come together for a joint investigation. Despite Ji Seok's jealousy, the joint investigation team switch partners. Ji Seok and Ma Ri become one team, while Ryung and Woo Hyuk become another. Meanwhile, Ji Seok decides to let Ryung transfer to a different team. As Ma Ri approaches him again, Ji Seok feels confused.

A rumor circulates about people disappearing after they party all night on Club Day, then taking the first train home. Ryung and Ji Seok get on the first train to find out whether the rumor is true. During their train ride, they run into a suspicious petty thief and the truth about the first train rumor comes to light. Meanwhile, Ryung finally meets the Subway Ghost a month after joining the Subway Police.

Ji Seok makes up his mind to kick Ryung out of the Subway Police for taking investigations into her own hands all the time. However, Ryung is attacked while out on an investigation by herself and there is something weird about the man who seems to be following her. Meanwhile, the Wangsoori Subway Police get dressed up to go out on Club Day, the best day to boost their arrest records.

Ryung and Ji Seok save Choi Do Chul just in time. Throughout their investigation, Ji Seok miraculously learns to appreciate Ryung's over-the-top nature. Meanwhile, a star professional fighter becomes the PR Ambassador for the Subway Police. By chance, Ryung comes across his dirty secret and the Wangsoori Subway Police finally get a lead on the Grasshoppers.

Ryung and Ji Seok attend the disciplinary committee hearing and return to the station for night duty. Thanks to Ji Seok's martial arts demonstration and a pot of noodles, Ryung gets her hands on the screen door keys once more. Meanwhile, an ordinary father, whose livelihood takes place on the subway, goes missing.

Ryung officially becomes a member of the Wangsoori Subway Police and Ji Seok's partner. Ryung gets into trouble from the start, because she sneaks into the tunnels. Meanwhile, an elderly man who delivers packages via subway becomes the victim of an assault. Ji Seok realizes that the package is related to criminal activities. While Ji Seok wants to hand the case over, Ryung acts before she thinks, as always.

The Subway Police of Wangsoori Station is led by Chief Inspector Ko Ji Seok. He is the best of the best, although his only goal in life is to have a stable, uneventful career. However, a serial killer, nicknamed "the Subway Ghost", strikes the Subway Police 30 Year Anniversary event. An applicant shows up for a job interview to join the team.