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 it's too late?