What would you do if a friend you met on holiday turned out to have a dark secret? That’s the tricky situation that Peter and Debbie find themselves in.

While the pair were on holiday, they met Elsa, an American with strong views. A few months later, Elsa invites herself to stay at Peter and Debbie’s house. But who is Elsa?

Once Peter and Debbie check her identity online, they discover some shocking reports. Is she really a serial killer? Still, that’s no reason to kick her out, is it? The Unfriend explores what happens when you’re trying to protect yourself, all with a polite smile on your face.

The Unfriend is written by Steven Moffat, who has created 40 episodes of Doctor Who, starring David Tennant and Matt Smith, as well as Sherlock starring Benedict Cumberbatch and Martin Freeman.