Lee Yool falls off a cliff and nearly dies in an attempted assassination. He loses his memory, and wanders for 100 days under a new name and personality. During this period, he meets Hong Sim, head of the first detective agency in Joseon.


Detectives from the present and a detective from the past communicate via walkie-talkie to solve a long-time unsolved case.

Je-In is a mystery novel writer. She chases after a serial killer with her longtime friend Rok-Hwan. Rok-Hwan is a detective. A handsome and mysterious man, Jason, appears in front of them. 

On the day the 15-year statute of limitations expired on a high profile serial murder case that claimed the lives of 10 women, a family member of the victim throws himself off the building in front of Choi Hyeong-goo, the detective in charge of the case. Two years later, a man called Lee Doo-seok publishes a book titled "I am the Murderer" claiming responsibility for the murders that took place 17 years ago. His book becomes a bestseller because of the author’s charming looks and his vivid depiction of the crime. The detective and the self-proclaimed murderer meet face to face on a television debate show. Believing that Lee is nothing but a con artist, Detective Choi renews his search for the ultimate truth. Then another man appears suddenly, claiming he is the real killer.