Peter, a thirteen-year-old loner, is forced by his mother in England to spend two weeks of holiday with his recently widowed aunt on the Irish island of Inishbofin, or the Island of the White Cow. On the ferry across, he meets Siobhan. She’s a year older. The island is suddenly struck by two tragedies, as two islanders have gone missing. On a walk, Peter meets two small men, who turn out to be the Irish fairy leprechauns. They inform Peter and Siobhan that they too are changelings, having been substituted at birth for normal babies. It is a time of great evil, and the Miasma of Evil is moving on to God-fearing citizens. The Miasma has recruited the terrifying Irish fairy horses, or Dullaghans, with their headless black riders. Peter and Siobhan become leaders in the fight, together with historic characters, including the sixteenth-century female pirate Grace O'Malley. They are transported by a Time Rail, which places them in the sixteenth century, where Peter has to rescue himself from Grace’s pirates.

The battle against the dreaded Miasma is getting close. Siobhan is found to be the incarnation of one of the Irish fairy’s greatest warriors from the past. In a frightening climb on a storm-ravaged rock in the ocean, Peter has to rescue a magic harp to help defeat the evil invaders. Merriline, the great wizard, is seeking books of necromancy and their spells to defeat the Miasma. Siobhan has to fight the Miasma chief.