15x08 The Invisible Enemy Part Four
That was that, then. Not a very well-regarded story (it came 196th in DWM's recent poll) and it doesn't particularly deserve to be either. Some good bits though.
Duck Quacks Don't Echo
2x01 Episode 1
Paul O'Grady: For the Love of Dogs
3x01 Episode 1
[Watch it (again) on ITV Player.]