And so begins David Tennant’s final run as The Doctor before he’s thrown in at the deep end, and it’s appropriate that for such a bombastic, energetic and fast-moving Doctor, the story demands that he’s immobile, frustrated and unable to help.

It’s actually a very old-fashioned story – Space 1999 meets ‘Horror Of Fang Rock’, with a bit of 28 Days Later mixed in – and impressively, since Russell T Davies has always defiantly and gloriously written primarily for the kids rather than the fans, it’s also quite an adult story, with David Tennant’s Doctor questioning his entire character and motives. In this story, he is not a hero, not a saviour. In this story, he simply has to walk away.

In style, it harkens back to the very early days of Who – Hartnell and Troughton pottering around gleaming white space stations – but in theme, it’s a lot closer to something a lot bleaker: it’s not guaranteed that The Doctor will win, or indeed survive. In fact, if we hadn’t already seen Matt Smith’s bow tie (which some of the younger viewers won’t necessarily have done so), this really could seem like the point where it all ends.

It’s not the most cheery of stories – indeed, the ending takes a beat right out of one of the most depressing Stephen King adaptations in recent years – and RTD dispenses with most of his customary humour (although there’s a cute line when The Doctor is asked his name, rank, and intention: “Doctor… Doctor… fun…”).

It’s a bit of a risky manoeuvre: if David Tennant’s not saving the day, or at least making a joke about it, then we have to rely on the story and the effects. Luckily, the effects are very simple, but very effective – there’ll be a good few kids wetting their bed after seeing the already iconic looking creatures that populate this story.

‘The Waters Of Mars’ is a story that essentially reboots Tennant’s Doctor before we lose him for good. Up until now, the emphasis on his Lonely God has always been on the lonely bit. In this episode, he wields the power of a God, without realising that he’s already part of history – without his influence, the team would have gone back to Earth, unaware and infected – and washes away his doubt (with an extraordinary line that fits the tenth Doctor’s arrogance:

“The laws of time are mine – and the obey me!”) before being brought down to Earth in one of the bleakest endings the series has had.

It all sets us up promisingly for the Christmas and New Year specials.

Aired at 7pm on Sunday 15th November 2009 on BBC One.