A man and a woman who were each other's first loves during childhood reunite years later as adults.
In his childhood, Yoon Na-moo was a seemingly quiet and mysterious boy whose father was a psychopath serial killer. He meets his polar opposite in Gil Nak-won, a daughter of a popular actress. Through Nak-won's persistence and pure and loving nature towards Na-moo, Na-moo falls into a reciprocated love. But when Na-moo's father, Yoon Hee-jae — a man with a twisted affection for his son — murders Nak-won's parents, their short love comes to an end.
Years later, Gil Nak-won is now Han Jae-yi, an aspiring actress (like her mother) who always tries to have a good outlook on life, but who also has a panic disorder due to her traumatic past. Yoon Na-moo is now Chae Do-jin, a rookie detective who wishes to atone for his father's sins. These two would-be lovers have held on to the precious memories of when they were together, but, connected by a tragic fate, they must try to overcome the stigma and hardships they will later face.[4]
A rookie detective who graduated from police university at the top of his class. He has never forgotten his first and only love, Nak-won, for whom he felt guilty since his father killed her parents. He later becomes Jae-yi's boyfriend.
A woman with a post-traumatic stress disorder who dreams of becoming an actress like her mother. Though she has suffered greatly, she overcomes it and vows to live well. She never blames Do-jin despite him being the son of her parents' killer. She eventually becomes Do-jin's girlfriend.
Chae Do-jin's father. A psychopathic serial killer who displays a twisted affection for his son and shows no remorse. He murders Jae-yi's parents and countless other people. After Do-jin calls the police, he is arrested and sentenced to death after the court finds him guilty of twelve murders. He later escapes from prison and commits a few more crimes (including two murders), and tries to mould Do-jin into a killer by kidnapping and killing Jae-yi. However, he fails, and Do-jin arrests him.
A prosecutor and Han Jae-yi's adopted brother who cares for his sister. He is wary and distrusting of Chae Do-jin, since Do-jin is the son of his foster parents' killer. It is later revealed that he is the murderer of his parents' killer, but he is acquitted due to his extremely young age, and because it was a killing done out of self-defence. He eventually trusts Do-jin, and in the final episode, he approves of his sister's relationship with Do-jin.
Chae Do-jin's elder brother. He has a violent personality and tries to follow his psychopathic father's footsteps. He feels jealous towards Do-jin for being his father's favourite child, which is compounded by the hatred he feels against Do-jin for reporting their father to the police.
Chae Do-jin's stepmother. She is Hee-jae's wife from a fourth marriage, and she treats Do-jin as her own child. She repeatedly tries to open up to Hyun-moo and accept him, but Hyun-moo rejects her kindness, until much later in the series when Hyun-moo finally opens up to her.
Chae Do-jin's younger stepsister. She is the daughter of Ok-hee and another man (prior to her marriage to Hee-jae), but she still regards Do-jin and Hyun-moo as her brothers despite having no blood relations to them.
The sole female police detective of Do-jin's unit, and Do-jin's former classmate in the police academy. She sympathises with Do-jin and befriends him despite knowing his father's identity.
A police detective who investigates Hee-jae's case and a mentor of Do-jin in the police force. He treats Do-jin with care and kindness, and loves Ok-hee but never confesses to her. He is later killed by Hee-jae when the latter escapes from prison.
An unscrupulous reporter who often makes news reports about Hee-jae's case, and harasses both Do-jin and Jae-yi for the sake of getting more exclusive news reports related to Hee-jae as an advancement of her reporting career. She is murdered by Ji-hong, who is earlier released from prison.
A female reporter who is Hee-young's rival. She also tries to get a scoop regarding the Yoon Hee-jae serial murders by approaching Do-jin and Jae-yi. Unlike Hee-young, she is more morally equipped and realises to an extent the negative impact the reports about Hee-jae's case have on Do-jin and Jae-yi.
An obese yet jovial and kind man who is Jae-yi's manager, and who cares about Jae-yi's welfare.
Jung Da-hye as Cheon Se-kyung
A famous actress who is Hye-won's junior prior to Hye-won's murder. She may look unfriendly and cold, but she is actually warm-hearted and caring to people around her, especially to Jae-yi whom she sympathises for her trauma of losing her parents.
Park Kyung-choo as Gil Sung-sik
Han Jae-yi and Gil Moo-won's father. He is a lawyer who is murdered by Hee-jae.
Han Jae-yi and Gil Moo-won's mother. She is an actress who is murdered by Hee-jae for treating his son well.
Hong Seung-bum as Lee Seung-woo / Yeom Ji-hong
An ex-convict who idolizes Hee-jae after meeting him in prison, and seeks to become a serial killer like him. After his release from jail, Ji-hong kills Hee-young as revenge for Hee-jae, and even assaults several people in different secluded places.
^ abIn order to circumvent Korean laws that prevent commercial breaks in the middle of an episode, what would previously have been aired as single 70 minute episodes are now being repackaged as two 35 minute episodes, with two episodes being shown each night with a commercial break between the two.[1]
^No episodes were aired on June 7 due to coverage of the FIFA Friendly soccer match between South Korea and Bolivia.
^No episodes were aired on June 20 due to live coverage of the 2018 FIFA World Cup Group B match between Portugal and Morocco.
^No episodes were aired on June 27 due to live coverage of the 2018 FIFA World Cup Group F match between South Korea and Germany. Aired 30 minutes earlier due to live coverage of the 2018 FIFA World Cup Group H match between Japan and Poland.
^Jung, Yu-jin (December 31, 2018). 소지섭 ‘데뷔 23년 만에 첫 대상’(mbc연기대상)[포토엔HD] [So Ji-sub Soo'First Debut in 23 Years' (mbc Drama Awards)[Photo and HD]]. Newsen (in Korean). Archived from the original on December 31, 2018. Retrieved June 5, 2020 – via Naver.
