Hye-ja is a single mom to 27-year-old Do-joon. Her son is her raison d'etre. Though an adult in years, Do-joon is naive and dependent on his mother, and a constant source of anxiety, often behaving in ways that are foolish or simply dangerous.

Walking home alone one night down a nearly empty city street, he encounters a young girl who he follows for a while before she disappears into a dark alley. The next morning, she is found dead in an abandoned building and Do-joon is accused of her murder.

Thanks to an inefficient lawyer and an apathetic police force, Do-joon's case is quickly closed, but his mother refuses to let this be the end of the story. Trusting no one, Hye-ja's maternal instincts kick into overdrive, and she sets out to find the girl's killer and prove her son's innocence.