Wangsuri Subway Police Investigation Team 1's Captain Ko Ji-seok graduated as the valedictorian of the police university. But his only goal is a stable police officer life. One day, an event turns Ji-seok's peaceful life upside down.

Yoo Ryeong, Ji-seok's new partner and the rookie detective of the Wangsuri Subway Police, enters the tunnel and gets attacked. Now her job is at risk. An old deliveryman gets attacked at a station, and Ji-seok realizes it isn't a simple assault case.

"Do you want to eat noodles?" Ryeong and Ji-seok eat noodles in the night-duty room. Ryeong gets the chance to search the tunnel thanks to it. Meanwhile, Choi Do-cheol, a father and a husband who makes a living in the subway, goes missing.

The truth of Choi Do-cheol's case is revealed. Ji-seok realizes Ryeong is more than just crazy. Star fighter Kim Won-tae becomes the Subway Police's ambassador, but Ryeong uncovers his ugly truth. A crucial clue about the Locust Swarm is found.

Ji-seok decides to kick Ryeong out of the Subway Police. While investigating alone, Ryeong gets attacked by an unidentified assailant. Meanwhile, Wangsuri Subway Police prepare to catch perverts on "Club Day" but are stopped by an unexpected guest.

"A woman who takes the first train after clubbing disappears." Ryeong and Ji-seok take the first train to see if this urban legend is true. Meanwhile, Ryeong faces the Subway Ghost after one month of working for the Subway Police.

An unprecedented co-investigation of the Metropolitan Investigation Unit and the Subway Police! Ji-seok pairs up with Ma-ri, while Ryeong and U-hyeok become partners. Ji-seok dismisses Ryeong from the team, and Ma-ri takes the chance and makes a move.

Ryeong speeds up her investigation of the tunnel and finds someone waiting for her. Meanwhile, Park Jeong-hun, an autistic child, is kidnapped at the bakery in Wangsuri Station. To make matters worse, the Locust Swarm sweeps up a whole car.

Jeong-hun must be saved! Ryeong and Ji-seok work together to catch the kidnapper. Showing off their great teamwork, they find out the kidnapper's identity! Meanwhile, the Subway Ghost goes after the next target.

Finally, the Subway Ghost's hideout is revealed and his identity starts to come to light. Meanwhile, they find out about Ryeong's illegal tunnel investigation, making Ji-seok face the biggest crisis of his career.

The suspect of the subway serial murder case is narrowed down. Ryeong and the Metropolitan Investigation Unit start tracking down the Subway Ghost! Ji-seok's investigation instinct is awakened once again.

The Subway Ghost investigation has a setback as Han Tae-ung runs away. To make matters worse, Ji-seok is on the verge of being kicked out of the Subway Police. Ryeong devises a plan to save Ji-seok.

The Subway Ghost is finally caught! However, it isn't long before the Subway Ghost tells an unexpected and shocking story that no one ever imagined. The story behind the subway serial killer slowly unfolds.

The Subway Ghost is inside a mental health hospital and is preparing for another murder! An intense investigation continues in an effort to catch the Subway Ghost, and in the end, Ryeong faces the shocking truth.

Finally, the Subway Ghost's mysterious MO is revealed! Ryeong becomes hopeful when she finds out that Jin is alive.

Eventually, Ryeong and the Subway Ghost face each other! At the Subway Ghost's wrath, even Ryeong is on the verge of being killed. Another tragic incident that no one imagined happens, and when everyone is in shock, Jin comes back.