Season 1

Ji Sung, an NIS agent, is enraged and turns desperate after his colleague gets killed in Hong Kong after chasing Wang Wei. On the flight back, Ji Sung steals Wang Wei's passport. Subsequently, Wang Wei faints, swallowing a pill. Do Kyung says she'll pose as his fiancee, but Ha Jun, the monitoring team leader, asks her to go through immigrations. Ji Sung keeps an eye on Do Kyung walking to the transit area instead of immigrations. Later, Do Kyung and Ji Sung get arrested for a drug case.

Incheon Airport goes into an emergency state due to Wang Wei. Ji Sung asks Do Kyung and Ha Jun for help, claiming Wang Wei isn't a typical illegal alien. While searching through the baggage screening area, Ji Sung finds Wang Wei running toward the runway and a fierce chase takes place.

After taking shots at a work function, Do Kyung gets sick and Ji Sung helps her. In the process, the two talk and learn about each other. Later, Ji Sung demands that the airport raise the security level for an industrial spy at his first meeting with the airport authority and the board. Despite Do Kyung's resistance, the security level gets raised, causing delays for passengers. Being blamed for the commotion, Ji Sung finds a Triad pendant on the neck of a man who may be the spy.

After missing Charlie, the suspected industrial spy and underboss of Triad, Ji Sung lashes out at Do Kyung. Ha Jun scolds him for being harsh, but Do Kyung says she understands Ji Sung. Meanwhile, Do Kyung surprisingly bumps into Charlie. After a quick encounter, Do Kyung runs to get help but gets held hostage by Charlie. Watching the event at the monitoring room, Ji Sung rushes to the site and faces Charlie.

After a suicide attempt of the daughter of North Korea's top politician, the hospital becomes a special security area, and the NIS bans any access. This creates conflict within operation headquarters. Myung Woo hears from Hye Ryeon that she entered illegally to meet her lover, and she asks Ji Sung to help her. However, Ji Sung coldly refuses to. Meanwhile, Do Kyung freezes when she sees Yi Kyung, but she turns away and leaves. Later, Do Kyung looks for Yi Kyung again while Yi Kyung watches.

Runway Two is closed due to birds and operation headquarters try to devise a measure before the holidays. While Ji Sung and Myung Woo wait for a meeting, Do Kyung walks in on them and leaves, embittered. Later, Do Kyung finds Myung Woo waiting for the bus with luggage, and offers that she stay at her place. On the same night, Do Kyung follows the North Korean research team's advice and draws a big bird on the runway. Ji Sung offers help, but Do Kyung declines and sends him to Myung Woo.

Do Kyung gets arrested after carrying luggage with drugs for her friend, Jung Min. For this, Do Kyung's office and home are searched, and she goes through intensive interviews. Ji Sung gets frustrated as the only person that can prove Do Kyung's innocence is Jang Han Chul, who is in a coma after swallowing drugs. Later when Ji Sung visits Do Kyung, he ends up punching the investigator and gets suspended. Meanwhile, Ha Jun goes to Hong Kong to meet Jung Min and get Do Kyung released.

Do Kyung is temporarily released with the help of the CEO. After coming home, Do Kyung is pleasantly surprised as her home is tidy despite the prosecutor's raid, and a card beside a vase tells Do Kyung that Myung Woo cleaned her home, evoking in her mixed feelings. Meanwhile, Nan Young tells Do Kyung that Ha Jun left for Hong Kong to help her. After failed attempts to reach him, Do Kyung leaves for Hong Kong and Awi ambushes Ji Sung, who has been talking to someone on a breakwater in Hong Kong.

Do Kyung finally proves her innocence, but experiences severe reprimanding at work. Despite the staff worrying about her being discouraged, Do Kyung concentrates on work even harder. Meanwhile, Myung Woo finds out where Ji Sung is staying through the NIS chief, Jae Moo, and sees him suffering from a high fever in a remote island village. As the operation headquarters plans a picnic, Do Kyung proposes to visit the Island Ji Sung is staying at.

An error with the luggage system creates confusion at the airport. Trying to handle the issue, Do Kyung proposes that they use the luggage system in Terminal 2, which is not yet open. Meanwhile, Ji Sung asks Jae Moo to let him stay at the airport when Jae Moon tries to send Ji Sung abroad. Fortunately, Ji Sung manages to stay and starts tailing an expert note forger who enters Korea. At the same time, Do Kyung receives a call blackmailing her with a bomb in the airport.

After finding that Sergei, the supernote forger, contacted a Russian mafia before being killed, Ji Sung raids the mafia but fails to find a lead. As the case becomes harder to be resolved, Ji Sung finds out about airport banks receiving new notes from Hong Kong every day and demands to view the process. This brings Ha Jun to strongly resist Ji Sung for doubting the security team, but Do Kyung cooperates. Meanwhile, Do Kyung sees Myung Woo for a migraine and Myung Woo gives Do Kyung her blessing.

As the raised security level disturbs their operation, supernote forgers leave bags disguised as explosives within the airport. The airport experiences another commotion, and Ji Sung once again demands to see the dollar delivery, figuring out the suspects' intentions. During the turmoil, the fake notes are swapped with real dollars. Mr. Um, when passing by the crime scene, gets attacked by the forgers as he attempts to scold them for using the carts.

Myung Woo faints and Do Kyung takes her to a nearby ER per her request. Myung Woo also asks her not to alert the airport staff or Ji Sung. Later, Do Kyung becomes busy with a project to service a new airline. However, the vice president of the airline and a Korean gang boss the NIS is after enter via the same flight, and a commotion caused by the boss puts Do Kyung in a tight spot. Do Kyung asks Ji Sung to sort it out, and though Ji Sung jokes around, he resolves the issue.

Sun Woo holds Kang Hyung hostage and points a gun at Ji Sung. Michael, the vice president of Neo Airline, walks toward the scene, hearing the alarm, and Do Kyung distracts him by talking about emergency training in the security room. Meanwhile, Myung Woo tells Ji Sung about moving to another hospital with a better offer and a new man in her life. Ji Sung briefly congratulates her and Do Kyung comforts him, while Myung Woo watches them at a distance with sadness in her eyes.

Sun Woo escapes from NIS's protection and fights a gang that attacked him. Ji Sung manages to locate him, but in the process, Sun Woo disappears and becomes wanted. Meanwhile, Do Kyung gets anxious about not being able to reach Ji Sung, and Myung Woo warns her about the pain she'll have to endure. Eventually, Do Kyung asks Ji Sung to quit, but he comforts her by saying he's not in danger. Meanwhile, the headquarters find out about soaring Neo Airline's stock prices and tries to discover why.

When Do Kyung tells Ha Jun about Myung Woo's illness, he warns her not to tell Ji Sung, and that she must respect Myung Woo's dignity. Meanwhile, Ji Sung receives an order to leave for Russia and when he tells Jae Moo that he refuses to go, is slapped. Later, Ji Sung finds out about Myung Woo through Seok Hoon, and seeing her in a coma, cries in regret. At that moment, Do Kyung walks in with flowers for Myung Woo, and upon spotting Ji Sung crying, she senses an impending breakup.