For young Emily Callaway, her games of Hide and Seek with an imaginary friend named Charlie have become anything but simple and innocent. Instead, she finds herself in the middle of a series of increasingly nightmarish acts that even her father David cannot stop. Who – or what – is Charlie? David wonders. How can an "imaginary" entity have this kind of hold on her? Maybe Charlie is not imaginary at all, but instead a flesh-and-blood, malevolent presence?