When seventeen-year-old Brigid chooses a worthless family trinket as her "something old" to wear on her wartime wedding day, she has no idea what she has set in motion. Brigid knows nothing about the iron brooch, nor has she ever been to Scotland, her father's homeland, but before long, the brooch makes its power felt, as she finds herself drawn into the world of a book about the Celtic fae - and in particular, she senses the influence of its author, a 17th century clergyman called Robert Kirk. From the Blitz to the brooding landscapes of Highland Perthshire, a dual timeline twists and turns to a startling climax.