Based on Hitoshi Iwaaki's classic Japanese manga Parasyte, a hit in the 1980s and 90s, it also has a popular anime series and a two-part film in Japan. Director Yeon Sang-ho stated that 'Parasyte: The Grey' is in the same timeline as Hitoshi Iwaaki’s 'Parasyte'. The only difference is that it is a phenomenon happening in Korea, at the same time as in Japan.
Director Yeon thought to himself, 'What would Korea be like if it coexisted with parasitic beings?' Thus, Director Yeon wrote a letter to Hitoshi Iwaaki explaining that he wanted to create this new series from Parasyte. Director Yeon said during the press conference that the mangaka agreed and gave Director Yeon his permission to create the series and he would like to see Director Yeon’s adaptation.