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.

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.

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 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.

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.

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.

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.

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.

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.

The Subway Ghost’s hideout is finally uncovered. Through the evidence found in the hideout, the police slowly start to piece together the Subway Ghost’s identity. Meanwhile, Ryung’s illegal investigations in the tunnels come to the attention of the commissioner and Ji Seok faces the biggest crisis in his career.

The police make headway on the Subway Serial Murders and narrow down the suspect list. After getting a tip about the identity of one of the Grasshoppers, Ryung and the Metropolitan Detective Division chase him down as he might be the Subway Ghost. Meanwhile, Ji Seok’s investigative instincts start acting up again, despite his suspension.

Ji Seok and Ryung catch the remaining Grasshopper who everyone believes is the Subway Ghost. However, while being interrogated, the Captain of the Grasshoppers makes a shocking statement. The truth about the Subway Serial Murder Case finally begins to reveal itself and Ryung is hopeful that she may find out what happened to her sister.

The police believe the Subway Ghost is in a mental hospital and that he plans to commit another murder. They chase him down and search the entire hospital building. Amid the chaos, Ji Seok’s mom and her caretaker are nowhere to be found. The Subway Ghost leaves behind a video clip, revealing himself and the shocking truth.

Ryung and Ji Seok make their way into Kyung Hee’s house and find a room that looks like a young girl’s. After sharing the new evidence with Ma Ri and her team, they split up into two groups to chase both Kyung Hee and Yi Joon. Meanwhile, Ryung’s hopes of finding Jin alive begin to grow and Ji Seok gets a call from the hospital about his mom.

Finally, Ryung and the Subway Ghost come face to face. As his rage explodes, the Subway Ghost attacks Ryung, but another tragedy puts a stop to his actions in the end. Meanwhile, Jin appears at the scene, only to run away amidst the chaos. Ryung, the Subway Police, and Ma Ri’s team put in all their efforts into reuniting the twin sisters.