Hae-Young (Lee Je-Hoon) works as a criminal profiler and is also a police lieutenant. Even though he is a cop, he doesn’t trust cops due to his own past experience. Back in 2000, when Hae-Young was in elementary school, his classmate Yoon-Jung was kidnapped and later found dead. Hae-Young saw a woman taking Yoon-Jung from school that day, but the police announced that the suspect was a man. Hae-Young even went to the police station and told the police officers that he saw the kidnapper and she was a woman. The police officers though didn’t pay attention to him. The killer was never caught and her case remains unsolved to this day. Hae-Young meets a reporter at a cafe to reveal bombshells about a celebrity scandal. Meanwhile, veteran detective Soo-Hyun (Kim Hye-Soo) is at the cafe staking out the meeting, unaware that Hae-Young is a cop. She arrests Soo-Hyun for stalking. While leaving the police station, Hae-Young hears someone talking to him. He looks around to find a walkie-talkie. The man who is talking to Hae-Young is Detective Jae-Han (Cho Jin-Woong). The detective though exists in the year 2000 and is investigating the disappearance of Hae-Young’s elementary school classmate. Hae-Young and Detective Jae-Han begin to communicate via walkie-talkie.