Joon Kang, a motorcycle courier who knows Seoul's back alleys better than anyone, makes a living delivering packages for shady clients no questions asked. He's fast, invisible, and broke, trying to pay off his older brother's gambling debt to a local gang.

One rainy night, he delivers a small, sealed box to a luxury penthouse in Gangnam. The next morning, the CEO of a powerful tech company, Ha Sung-Jin, is found dead - shot with a custom gun matching Joon's package. His face floods the news, labeled the "Ghost Courier Killer."