Mariah Ellison isn't merely disappointed to learn that she won't spend Christmas at home with her married granddaughter: she is furious. Instead, Grandmama is being packed off to a house in the Romney Marshes to stay with her ex-daughter-in-law. Never having got on with Caroline, Mariah much disapproves of her new husband: decades younger than her, Joshua, an actor, is scarcely even respectable. There will be nothing to do, no one to visit, and no doubt the terrible weather will make even taking a walk impossible. It is going to be the worst Christmas of Grandmama's life.
As if that weren't enough, another visitor is foisted on the household. Then something shocking and quite unexpected happens. Has a crime been committed? Grandmama is surprised to find herself turning detective - another profession she deplores - and proving extremely good at it.