The story is set in contemporary South Korea. Some 30 years ago, unidentified creatures appeared out of nowhere and attacked mankind. They caused massive destruction wherever they went. The monsters are called "Troubles", and caused the world to fall into chaos. Along with the first appearance of the "Troubles", some number of humans were born with or acquired supernatural abilities. These mutants called "Traces" are the only ones who can fight off the Troubles. The Traces are also considered freaks and generally shunned by the community because some of them misused the powers for their own interest.