In present-day 2015, criminal profiler Park Hae-young (Lee Je-hoon) finds a mysterious discarded walkie talkie; inexplicably, the person at the other end, Detective Jae-han (Cho Jin-woong), is from the past, and addresses him familiarly, even though Hae-young has never met him.

With the case still ongoing in 1997, Jae-han demands to know who it is, but Hae-young refuses to tell him to avoid the consequences that come with changing the past.

Jae-han had continued investigating and arrested Jin-woo in 1998 all on his own, thus preventing the nine additional murders that had occurred since 2001; each of those victims are now alive and well.

Jae-han is helpless when he learns that the police themselves bribed the detectives and witnesses to cooperate with Sun-woo's framing.

Realizing he must be dead, Soo-hyun resolves to visiting the forensics center each time they receive skeletal remains to see if it is Jae-han.

As a teenager, Hae-young remembers being told that Sun-woo was framed due to being poor and weak, so he resolves to become strong and educated.

Upon its premiere, the series attracted attention for weaving actual cases into its plot and attained high ratings.

[13][14] It received praise for its solid acting, tightly-constructed plot and detailed and sophisticated direction; and enjoyed success internationally in China and Japan.