The veteran detective Seo Do-cheol and his team at Major Crimes, relentless in their pursuit of criminals, join forces with rookie cop Park Sun-woo to track down a serial killer who has plunged the nation into turmoil.

During the Japanese occupation of Korea, the Japanese Empire seeks to eradicate the Korean language and identity. In retaliation, a small group of Korean patriots try to protect their language by compiling the first Korean language dictionary.