There is magic deep within Arranmore Island, and Fionn Boyle is beginning to discover how it has woven its way through generations of his family's history. But Arranmore is in trouble; evil sorceress Morrigan's soul stalkers have returned, giving rise to widespread fear and suspicion. Fionn wants to help, but the Storm Keeper magic passed down from his grandfather seems to have deserted him.

Fionn sets out to summon the merrows, a vast army of sea creatures who may be his only chance. But how can he find them without the faintest idea of where to look? The battle to save Arranmore has begun.