It's Christmas 1909, and for once Lady Hardcastle - respectable gentlewoman, amateur spy - and her lady's maid, Florence Armstrong, are setting sleuthing aside. They are invited to the festivities up at The Grange, as guests of Sir Hector and Lady Farley-Stroud.
But barely have corks been popped and parlour games played when a mysterious crime comes to light. Someone has broken in while the revellers were distracted and made off with a priceless pearl necklace. Lady Hardcastle and Flo are determined to catch the thief - but with so many Christmas guests encamped at The Grange, is it possible that the felon is hiding in plain sight?
With the clues stacking up, Lady Hardcastle bears down on her culprit. But just as the pieces come together, it begins to look as if there is something more devious afoot at The Grange...