中文简介：

卡西的哥哥拜伦误入了一群野蛮鲁莽的男孩中，很快发现他们根本不是人类，而是Tylwyth teg精灵，他们引诱人类进入世界的黑暗之地。拜伦也被诱惑了。

拜伦失踪后，卡西和表妹西恩跟踪他的踪迹来到一条废弃的老铁路隧道，发现它通向地下世界Annwn。在那里，他们发现Tylwyth teg精灵异常不安分且愤怒。他们的领袖Gwenhidw想保护Annwn免受人类破坏，并将拜伦作为她计划的一部分。但卡西绝不会让任何人利用她的大哥。她能及时救出他吗？

英文简介：

Cassie's older brother Byron has fallen in with the wrong crowd – it's soon clear these boys are wild, reckless and not 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, Siân, follow his trail to an old abandoned railway tunnel which goes down and down 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. Byron is part of her plan. But Cassie won't let her big brother be part of anyone's plan. Can she rescue her brother before it is too late?