The Christmas adverts have come out in force. After reading Charlie Brooker's excellent piece on the latest awful crop of "seasonal" advertising I've found myself with the theme from the Morrisons advert stuck in my head. I'm blaming you Charlie.

One advert that at least 9 Million people watched last night was on the BBC. Doctor Who had a sixty minute pitch for its apocalyptic Christmas episodes. It was intriguing, somewhat baffling and pretty terrifying. I'm not going to attempt to filter my following comments for spoilers, so go away if you haven't watched and don't want to know what happened. That was your only warning.

Why do I describe the episode as an advert? It certainly wasn't light bright and fluffy. But I wouldn't mind betting that the key word in the preparatory tone meeting for this episode was "Foreboding". We were foreboded in spades. From the early moments when the Doctor says "I shouldn't be here" we were left with a sense that something is coming. In the early moments of the episode we're shown obituaries for all the guest characters to flag up early that things won't end well. Several times throughout the action the Doctor's face is clouded with doubt, as if he's wrestling with his conscience. He wants to save these folk, but knows that he mustn't.

The actual threat faced by the Martian settlers isn't explained. At the end of the episode we have no more idea what it was that the broken water filter let through. All we know is that it turned crew members into zombies and that it was highly infectious. The story wasn't about this particular struggle; it was about the Doctor. He's clearly been travelling alone for some time now with no one to check the god complex that's been bubbling along under the surface since his last regeneration. We saw aspects of it back in The Christmas Invasion when he was clear about his "No second chances" rule. In the same story he brought down Harriet Jones' government with six words: "Don't you think she looks tired?", even though he'd previously described her premiership as a golden age that lasted three terms. In New Earth he declared that there was no higher authority than himself. There are many other examples throughout the tenth Doctor's era.

We need only look to later Christmas specials for more portents of the Doctor's superiority complex. In his first meeting with Donna Noble she begs him to find someone to travel with him because he needs someone to stop him from time to time. On the starship Titanic, Mr Copper sees the Doctor's expression at Rickston Slade's survival and notes that if he could choose who lived and died that would make him a monster. This is a line he crosses in Waters of Mars, and it's terrifying. The nightmare stuff with the water zombies is trivial compared with the prospect of an all powerful Doctor and a pitch perfect delivery by Tennant made the hints at what's to come seem most distressing. The Doctor has crossed a line and there's no going back.

There was an echo of a similar decision made last series when the Doctor discusses his time in Pompeii, when Donna begged him to save someone, anyone to make their time there have some meaning. We're left with the impression that saving Caecilius' family was the right decision, but how that coloured the Doctor's snap decision to save the three remaining crew members of Bowie Base 1 makes me wonder in retrospect. Were we reminded of that episode to show the contrast between saving "some little people" compared to saving Adelaide Brook? Or was it to remind us that the rot set in much earlier?

We're left with no doubt that there will be consequences for the Doctor's actions on Mars, and that these will inevitably lead to the death of the tenth Doctor. Although I'm also left wondering about some of his previous actions too. What about his changing of history to curtail Hariet Jones' government? What about triggering the Cyber invasion of The Next Doctor by sending cybermen and daleks tumbling through the void in Doomsday? His dismissal of the appearance of teh Cyber King in Victorian London with a simple "Funny That", seemed loaded at the time. Have we been witnessing a Doctor out of control for years, making a swiss cheese of time with catastrophic consequences? He said in this episode that his theory of fixed points in time with the rest being flux was "only a theory". What if every change has had repercussions? What if Time has only just been able to absorb the Doctor's actions for so long and now the straw has broken the camel's back? The Christmas double episode bears the weight of some very high expectations indeed.

Casual viewers got a bit of a run around with some water monsters and beautiful martian vistas. Fans were rewarded with a deeper story that challenges the heart of Doctor Who itself: What if the worst monster the Doctor ever came across is himself?