In the sweltering summer of 1915, Pin, the fourteen-year-old daughter of a carnival fortune-teller, dresses as a boy and joins a teenage gang that roams the famous Riverview amusement park, looking for trouble.

Unbeknownst to the well-heeled city-dwellers and visitors who come to enjoy the midway, the park is also host to a ruthless killer who uses the shadows of the dark carnival attractions to conduct his crimes. When Pin sees a man enter the Hell Gate ride with a young girl, and emerge alone, she knows that something horrific has occurred.

The crime will lead her to the iconic outsider artist Henry Darger, a brilliant but seemingly mad man. Together, the two navigate the seedy underbelly of a changing city to uncover a murderer few even know to look for.