Thea Stilton and the Thea Sisters are on an expedition to the fantastical Land of Erin! While they are searching for a missing mouse, they learn that the land is in peril. On their journey, the mouselets encounter fairies, pixies, gnomes, and other magical creatures--some helpful, but some out to trick them! Can they complete their mission and save the Land of Erin before it's too late? It's an adventure full of mystery and friendship!