The opening is probably the best hook this season: a man is strangled in a wood-panelled corridor; cut to (apparently) Nyssa asleep in a grand old bed. If this was a new series episode that would make an awesome teaser. It’s followed by more intriguing mystery: why does the TARDIS keep arriving on Earth? Why is everyone staring at Nyssa? And why is the Doctor expected at Cranleigh Hall? When Sir Robert mentions the Master, the Doctor is immediately on edge. A murderer on the loose, and the Doctor seems to have been drawn to the scene of the crime…

None of this actually goes anywhere, though. After the set-up, it’s mostly dancing and eating, while Davison gets stuck in a dressing gown wandering round secret passages. The script is quite funny (and naughty – ‘He was Smutty at school’ and ‘What do you do with a cocktail in a bath?’), and, just like K9 and Company, Terence Dudley writes believable, likeable characters. Sir Robert in particular is delightful: his flirtation with Tegan is a hoot. But he hasn’t got a clue where to go with the story, and the result is an episode with all the sense of urgency of a lazy summer afternoon.

The double plot at least gives Sarah Sutton something to do – it’s a shame Ann has to be essentially the same aristocratic character as Nyssa, rather than say a parlour maid (which might have given Sutton more opportunity to flex her performance), but the story relies on them being not only physically but socially interchangeable. And for all she’s dismissive about it now, Tegan finally looks like she’s enjoying time travel. Matthew Waterhouse is again left with the dregs, but he plays the script as if Adric has a crush on Nyssa (his face as he watches her dancing) – it’s a shame that undercurrent went untapped, because there’s something there in The Keeper of Traken and his joy at being reunited with her in Logopolis that’s never really picked up. It’s such a perfect character touch that Adric has pinned his gold star on his fancy dress costume.

Next episode: Black Orchid – Part Two