The Unfriend by Steven Moffat

When Peter and Debbie meet eccentric American Elsa while on a cruise, they never imagine she’ll take up their invitation to visit. But months later she turns up on their doorstep. Oh, and Elsa might be a murderer. But how do you uninvite someone without being rude? A hilarious comedy of manners by Doctor Who and Sherlock writer Steven Moffat.