What a gripping episode of Doctor Who. It was one of those episodes where they seem to have written themselves into a hole with a story having ramifications for all of time and space; now we have to wait until November to find out the denouement.

This is of course a little frustrating but perhaps these relatively short series with long gaps (along with a very high standard of writing) is part of the secret of how the BBC has kept the programme rolling forward for so long and, since the 21st century revival, going from strength to strength.