'The Unfriend' by Steven Moffat

While on holiday, Peter and Debbie befriend Elsa: a lusty, Trump-loving widow from Denver, USA. She's less than woke but kind of wonderful. They agree to stay in touch because no one ever really does, do they?
When Elsa invites herself to stay a few months later, they decide to look her up online. Too late, they learn the truth about Elsa Jean Krakowski. Deadly danger has just boarded a flight to London! But how do you protect all that you love from mortal peril without seeming, well, a bit impolite? Because guess who's coming... to murder!

This production contains some strong language.