(Again, I'll give no spoilers for those who haven't seen it...)

Well, sorry to gush (and I only really do that with certain things - most TV is bloody awful these days), but I'm actually thinking this might be one of Tennant's best episodes. Back in the 80s, Peter Davison had his best story right at the end, The Caves Of Androzani. It was a dark and scary story which ended with him saving his companion and regenerating. The Doctor acted quite out of character for much of the story and it was all-round brilliant. (It recently got voted best DW story of all time.)

Waters Of Mars is very similar in tone to that, but goes more to the heart of the character of the Doc and sees what would happen if he goes over the edge. I loved the atmosphere which was very much like an Alien film, the characters were just well-rounded enough without giving them too much weighty back-story and I even liked the deliberately-naff robot.

But the best thing of all is the Doctor himself. Tennant's performance was as good as he's ever been, just enough humour in the right places and completely manic when he needs to be. This story points out a few things:

1. The Doctor needs a companion. But more than that...
2. The Doctor needs the Time Lords top stop him going too far. But when he does go too far...
3. He's crossed the line and frankly deserves his coming regeneration. And I never thought I'd say that about any Doctor, particularly this one.

This is the sort of story that would have worked really well at the end of Chris Eccleston's run, but the fact that it's happened with Tennant makes it all the more dark and tragic. I reckon if you didn't like this story, you'll reeeeeeeally not like the christmas episodes.

That was certainly a fairly major plot-hole, if I'm honest. The Doctor goes on about these supposed "fixed-points in time" that have to happen so that the future can carry on as normal - then he goes loopy, ignores his own advice and alters a pretty major event. He's damn lucky that time sorted itself out an the end. And all this after he shows the consequences of Rose saving her own dad's life back in Eccleston's series! That was a plot-hole you could park a Tardis in.

Unfortunately, Dr Who is a series that's had a somewhat shaky relationship with its own continuity - it contradicts itself all the time. You wouldn't believe the amount of alternate histories the Daleks have had over the decades...