Headlong

The year is 5343 but Christmas street decorations are still those big bulbs made up of lots of little white lights, that turn up in high streets looking slightly tattier every year. Also, there's Christmas tree bulbs instead of planets in the opening credits NOW LET US NEVER SPEAK OF THIS AGAIN.

"The Husbands of River Song" by Steven Moffat, directed by Douglas Mackinnon.Spoilers after the cut.