Cassie's older brother Byron has fallen in with the wrong crowd; boys who are wild reckless and not even human at all. They are 'tylwyth teg' - Fair Folk - who tempt humans down into the dark places of the world. And Byron is tempted. When he goes missing Cassie and her cousin Sian follow his trail into Annwn the underworld. Here they find that the tylwyth teg are restless - and angry. Their leader Gwenhidw wants to protect Annwn from the damage humans are doing to the world and Byron is part of her plan. Can Cassie rescue him before it's too late?
