In a modern world of straight lines, these enigmatic circles in the landscape seem to call out to us from an earlier time. Fairy forts are the most numerous ancient remains in Ireland, numbering 50,000 or more. The traditions connected to them are so powerful that, even today, people don't interfere with them, aware of the terrible consequences of doing so. Roads curve around them; farmers don't plough over them. But what are they, who built them, and why? And how have so many survived untouched for so long? With evocative photographs by Richard Mills, master folklorist Jo Kerrigan explores the origins of these mysterious circular structures: the stories and legends, the history and archaeology, the living traditions and practices connected to them. And reminds us of the dangers of disturbing the ancient spirits dwelling beneath.