Watcher is a gritty psychological thriller that dives deep into the corrupt underbelly of the police force. Fifteen years ago, a tragic and mysterious incident destroys the lives of three individuals: Do Chi-gwang, an elite policeman who witnessed his partner's corruption; Kim Young-goon, a former police officer who witnessed his father murder his mother; and Han Tae-ju, a brilliant prosecutor who lost her finger and her sanity during the investigation. Now, fate brings them together as part of an internal affairs corruption investigation team. As the trio digs deeper into cold cases and current institutional wrongdoing, they realize that a powerful, shadowy cartel of criminals, corrupt cops, and high-ranking officials has infiltrated every level of law enforcement and government. Working together while harboring deep-seated distrust and their own personal agendas, they must uncover the truth behind the historic tragedy that inextricably linked their lives, forcing them to question the very nature of justice and who truly watches the watchers.