Magic is in the air when Allie and Jay Cameron visit their ancestors' ancient Castle Keep in Scotland, tucked in its unspoiled loch. The twins wake the mischievous shape-shifting Boggart and his infamous cousin Nessie, of Loch Ness fame. But a summer of fun-loving trickery with the Old Things is invaded by a dangerous real estate developer called William Trout.

Trout has big plans for a luxury resort on the loch, and little care for its people or the law. Bulldozers get to work. The future of the loch, its seals, and all its beauty are threatened. The twins and Angus Cameron, their grandfather, mobilize to save his shop and the loch, but it's soon clear they will need help of a different sort...

In a race against time, the Boggarts recruit help from other Old Things of Scotland: hair-raising creatures of the Wild Magic. But are the Blue Men of the Minch and the Nuckelavee too terrifying for humans to handle? How can they drive out the invader? What's certain is that Mr. Trout is in for a wild ride in this comical, page-turning adventure from Newbery Medalist Susan Cooper.