Signal

Profiler Hae-young finds an abandoned old walkie-talkie ringing inside a dump truck, parked outside a local police station; he hears the voice of a detective, who gives him evidence of an unsolved case of 15 years back.

Hae-young and Su-hyeon join the "Cold Case Task Force" which investigates the Kim Yun-jeong kidnapping case; Jae-han, tells Hae-young some clues from the year 2000.

Hae-young now realizes that Jae-han is from 2000, and that some consequences had changed the present day due to the clues from 2000; on a different note, the team works on the Gyeonggi Nambu serial murder case.

"Cold Case Task Force" continues the unwanted hunt for the serial killer from 2000; they finally have some substantial evidence in their hands.

Hae-young finds an old document that indicates Jae-han is missing; he continues to receive messages through the walkie-talkie.

Hae-young finds out the whereabouts and motives of the kidnapper of Sin Yeo-jin.