Wow. This one was, as had been suggested by the reading I'd done, a really good episode. A masterpiece of sound design being used to create an upsetting tone that builds to a crescendo.
Having woken from a disturbing dream, Angie Truscott (Elizabeth Sellars) is perturbed by scratching coming from within the walls and under the floorboards. Her husband Roger (Anthony Bate) arrives home and offers a rational explanation for the noises. With the radio on to hide they noise, they hear Barty's Party, an early evening drivetime type show which starts to take reports about large numbers of rats on the move.
This is essentially a two hander theatre piece, although with a few voices added such as the radio presenters and a telephone call, so it's vital that the two actors get it right and indeed they do. Elizabeth Sellars is particularly good, as you might imagine given her pedigree and she carries more of the piece than Bate. It's nice though, after the male leads in "Baby" and "Special Offer" being such nasty pieces of work, that despite occasionally looking like Roger would join them, he usually prefers to comfort his wife rather than dismiss her.
As I said in the opening paragraph it's the sound design that really does the best job of the episode. From individual scratching, building across the run time to sound like a hoard of scratching screeching creatures, it really helps your brain fill out what you can't see happening.
It's a great episode of television in any era.