We get a wonderful script - credited to David Agnew (a BBC pseudonym) but mainly written by Douglas Adams. A exceptional supporting cast: Julian Glover as Count Scarlioni aka Scaroth, last of the Jaggaroth is the main villain of the piece. He's intelligent & dangerously charming & Julian Glover is such an excellent actor that he manages to make Scaroth frightening without ever having to raise his voice. He's a master of delivering threats that sound like polite requests.
Then there's Catherine Schell as the Countess & a very beautiful woman...probably and Tom Chadbon is Duggan, a private detective with a penchant for violence against people & windows. The interplay between the Doctor, Romana & Duggan is blissful to watch, especially the Doctor's frustrating with Duggan continually knocking people out whilst in conversation.
Some might criticise David Graham as Kerensky, the Count's pet scientist for a frankly ridiculous accent* but he hits all the right notes in my book & the when the Count says that he can't believe that such a large intelligence can live in such a small mind he hits the nail on the head. Kerensky is a classic Doctor Who character, the scientist too focused on his work to ask - or perhaps care - about what that work might lead to.
[19/12/2013 Update: In fact the short conversation between The Doctor & Kerensky is a rather lovely little lecture on scientific responsibility. It's actually much more meaty than its tone might make you think.]
There are nice performances from people in minor roles to: Kevin Flood as Hermann the Butler is a masterclass in polite psychopathy. [Jeeves with a pistol] We even get a little appearance by the wonderful Peter Halliday as a bemused renaissance guard with cold hands. Not to mention the best cameo in Doctor Who history in the final episode when John Cleese & Eleanor Bron make a very minor appearance as art critics.
Then there's Paris. The first story filmed outside the UK & having the Doctor run around the streets of Paris, instead of London is nice, even if there's an element of 'Wish You Were Here' about making sure we see all the sights. Just to prove it is Paris. It does feel sometimes as if they we are watching 'guerilla filming' as some shots seem to have been done on the cuff, e.g. the scenes on the Metro in episode one.
I was going to say that the problem with that of course is that it makes the studio scenes seem a bit flat but it doesn't. The only scenes that don't really work in the studio are those in the cafe when a series of morose gun wielding fedora wearing grunts hold up the Doctor, Romana & Duggan without anyone raising the alarm. They are probably the only dud moments in the story.
[19/12/2013 Update : I think the thugs dress sense is based on a lot of French policiers from the 60s & 70s like Le Cercle Rouge & Le Samourai but I'm prepared to be corrected.]
Finally we have two pitch perfect performances by the leads. Tom Baker & Lalla Ward seem to be having the time of their lives, relishing being both in Paris & having a great script to work with. Lalla Ward in a schoolgirl outfit is also a pleasure, although whether the modern series would do this is an interesting point.
[19/12/2013 I'd stick to that judgement to some degree, although perhaps less so after this watch. It seems to me there's a gaping plot problem here, which is if Scaroth succeeds then he's surely creating a time loop. He goes back in time, stops his ship exploding & his people get away but if he stops his ship exploding he can't have been splintered in time and built a time machine in a 1979 Parisian basement. That means he can't have gone back in time to save his ship, which means his ship still explodes splintering him in time ad infinitum. This timey-wimey stuff isn't quite as straightforward as it looks.
But it's still great fun, eminently quotable & rather beautiful in its way. Whatever happens to Doctor Who, we'll always have Paris.]