The series focuses on the eponymous fictional webtoon entitled W, written and illustrated by its legendary author and illustrator Oh Seong-moo (Kim Eui-sung). W became so famous in South Korea that even its printed versions became a best-seller there.-
The storyline of the webtoon W starts with the younger years of its main character Kang Chul (Lee Jong-suk), a handsome and intelligent boy who is also skilled in shooting. He becomes famous in South Korea as he won a shooting competition during the 2004 Athens Olympics. But this fame is interrupted when the unknown Murderer massacres his family, and he is erroneously accused of the crime. Another villain, Han Cheol-ho (Park Won-sang), handles the case to gain political popularity, urging the court to impose death penalty upon Kang Chul. Despite his efforts, Kang Chul is deemed innocent of the crime. Depressed over losing his family, he attempts to commit suicide over the Han River but stops himself and chooses to search for the real killer instead and devote his life to crime-fighting.
Ten years later, Kang Chul becomes a multimillionaire. He starts "Project W" in an efforts to catch the real culprit behind his family's death; part of the project is his television broadcasting company dubbed "W," after the initials of the interrogatives "who" and "why" (hence, the title of the webtoon). One night, he receives a suspicious phone call and gets severely injured by the same mysterious Murderer at the rooftop of his penthouse.