Highlighting the fact that The Message-a 2009 Chinese espionage movie starring Zhou Xun and Li Bingbing-was a box-office success, Zhang says that, since then, few hit spy movies have been produced. As a result, he decided to delve into the genre after being contacted to direct Cliff Walkers.

Scriptwriter Quan Yongxian says the story is sort of a loose prequel of his 2012 TV series The Brink, about a couple of heroic underground Communist Party members.

Quan, a native of Jiamusi in Heilongjiang province, says he has a strong personal interest in the wartime history of northeastern China and how people there resisted the Japanese invaders.

As his elder sister worked at a local chronicle archive, Quan managed to read historical records, stumbling upon the thrilling details of a prison break from Zhongma Fortress in 1934. This was a death camp located in Beiyinhe village outside Harbin, where the Japanese army conducted biological warfare research on prisoners.