Thea Stilton and the Thea Sisters are on an expedition to the fantastical Land of Erin. While they are searching for a missing friend they learn that the land itself is in peril! The mouselets are ready to help. On their journey, they encounter fairies, pixies and other magical creatures some helpful, but some out to trick them! Can the Thea Sisters save the Land of Erin before its too late? Its an adventure full of mystery and friendship!