Applying critical faculties to a show about a 900 year old bloke bouncing round time and space in an old police box while chased by genocidal pepperpots requires a bit of selectivity if it is to be feasible, but here are some reasons why this episode is a Must Watch (mild spoilers, beware).

The whole programme was peppered with excellent dialogue: "I just made it up", "France. Another Planet.", "I'm not winding you up.". Madam de Pompadours delivery was perfectly aristocratic and the speech mannerisms were perfect. This was a woman with wit who could really string a sentence together. Mickey acts as the perfect "straight man" for delivering lines to. I hope they keep him around.

The costumes, the constantly shifiting locations each with lots of subliminal clues every time they are entered. The gorgeous costumes, and the absolutely fantastic image of the Doctor bursting out of the mirror - only an actual period drama series could do better.

Moffat continually played with our expectations. This was classic Who (and I use the word classic in a heavy cannonical fanwank old-series sense) in that we were put in the middle of a mystery and didn't fully know what exactly was going on until the very end. Revelation followed upon revelation, compounding the riddle: the fireplace, the automaton, the clockwork, Madame du Pompadour, the horse, the human heart, the mind meld suddenly turning two way, the zaphodic doc...at no point could I predict further than the next two minutes of the programme. If there was Nobel for television scriptwriting, Mr Moffat would be on the shortlist.

Easily the most subjective part, but Ms Myles blew away Ms Elisabeth Sladen from the previous episode. Sorry, Liz. David Tennnant managed an extraordinary emotional range, and so did she. The Doctors abandonment of Rose hightened the emotional tension of Rose and Mickey when they were trapped. Usually scenes like this have the impact of a wet lettuce because you know the Doc will wave his sonic screwdriver and save them, so these are my least favourite scenes but Billie acted her socks off with the only slightly - off dialogue of the whole script.

The dannana diaquiri lines are too derivative of Zaphod Beeblebrox (for a moment I thought I saw a second head). Easily forgiven. Then there's Arthur, the horse. There's a logical explanation for his presence, but blink - and you missed it. The Doctor finding his way back to the 51st Century via the fireplace. was unexpected, leading to a slight wiff of deus ex machina (or deus ex fireplace, as the case may be). There was no Checkhovian gun in scene one for this, unless that was another blink and you miss it moment. This being one of the frustrations of the 45 minute format: you have to be completely alert in order not to miss anything.

*swoon* *thud* 'nuff said.