Tir Na Nog is set in a mythical Celtic world peopled by the Sidhe. Once mighty, now fallen on hard times, these monkey-like creatures are the main protagonists in the adventure. They had bound the Great Enemy by creating the Seal of Calum, and ... had become a great civilisation. But the Great Enemy had managed to steal the Seal by sending a thief. In their rage the Sidhe killed the thief but the Seal was shattered into four pieces and the Great Enemy freed to wreak havoc on the Earth again. So fell the Sidhe into sub-human beasts.