A live-action remake of Yeon Sang-ho's 2011 animated film of the same name,[4][5][6] the series depicts a thriller drama of those who brought out memories of violence due to a mysterious serial murder that began with a message from a friend 20 years ago.

[8] Hwang Gyeong Min is a 35-year-old man who lives without forgetting the memories of school violence 20 years ago.

Jung Jong Seok is a 35-year-old detective who tracks messages from a friend 20 years ago.

Kang Jin Ah is a charismatic detective with strong principles and an obsession with solving cases.

The crux of the story shows dark secrets of Jong Seok and Hwang Gyeong Min's true nature as he tries to expose the truth behind Kim Cheol's death 20 years ago.