[1][2] Ahead of the premiere, a webtoon depicting why the series' main character, Cha Young-jin (Kim Seo-hyung), became a detective was released on KakaoPage.

[3] Still haunted by the death of her friend who was murdered 19 years ago by the infamous Stigmata serial killer, detective Cha Young-jin (Kim Seo-hyung) is determined to catch the culprit.

She is soon joined by school teacher Lee Sun-woo (Ryu Deok-hwan), and they do their best to protect the youth at risk, whose lives could have been changed if they had been surrounded by good adults.

One such youth is Go Eun-ho (Ahn Ji-ho), a sixteen-year-old boy who lives in the same apartment block as detective Cha Young-jin, and who seeks her friendship to avoid a traumatic home life.

The school he attends, where also Lee Sun-woo is a teacher, has close connections to a church and religious cult, and following a single act, saving the life of an old man (Kwon Hae-hyo) who collapsed in the street, he finds himself drawn into a web of intrigue, corruption and murder.