Matthew is a normal 11-year-old boy living with his parents and little sister in Surrey. He's too old and sensible to have an imaginary friend really. Yet when Matthew's parents keep finding him talking and arguing with a strange presence whom Matthew calls Chocky, that's what they believe it must be…at first. But Chocky is oddly sinister, and keeps asking Matthew all sorts of complicated questions about the world and making him behave in unusual and erratic ways. Then Matthew suddenly does something heroic, well beyond his capabilities; the media become interested and the interest in Matthew widens. His parents refer him to a psychologist. Who is Chocky? And what could he or she want with their son?