[2][3][4] The film is loosely based on the true story of Park Chae-seo, a former South Korean agent who infiltrated North Korea's nuclear facilities.

"[16] Maggie Lee of the Variety wrote: "Instead of the usual dose of action and suspense one expects of this genre, watching this dense 140-minute political drama unfold is like fumbling through a long tunnel that’s nonetheless worth it when the ray of light emerges at the end.

"[9] Deborah Young of The Hollywood Reporter described the film as "a stylish, blood-pounding thriller of the type Asian cinema is so good at making...For sheer topicality, the film is hard to beat, and to find a full-blown entertainment yarn in Cannes' midnight section that's partially set in newsworthy North Korea is rather astounding.

"[18] Yoon Min-sik of The Korea Herald wrote: "The scene of South and North Koreans sitting together and conspiring over what to do in order to retain power shows that things are not always so black and white.

Heukgeumseong, taught to regard all elements of the hermit kingdom as evil, sees the human side of the North Koreans through his interaction with them, extending his perspective to beyond what is presented to him...

"[20] James Marsh of the South China Morning Post rated the film 3.5/5 and wrote: "Rather than strive for a James Bond-style spy caper, Yoon explores his characters’ patriotic motivations, and in the process humanises many of the film’s primary antagonists, while exposing endemic corruption in South Korean politics.

Thankfully these moments are countered by some expertly executed sequences of high-stakes espionage, balancing the film’s even-handed politics with similarly assured thrills.