If action packed, edge-of-your-seat thrills is what you want from Doctor Who, then The Waters of Mars is satisfying stuff. It tells the story of Earth’s first colonists on Mars, systematically picked off by a sentient micro-organism which transforms its victims into water exuding monsters. History says that the colonists are doomed and the Doctor (David Tennant, nearing the end of his Doctordom) is torn between saving these legendary astronauts and allowing established events to play out as recorded. Directed by the show’s great galvaniser Graeme Harper, it’s fast, frightening and foreboding. It’s as good an example of that particular genre of Doctor Who as we’ve ever got and given that particular strain gives us The Seeds of Doom, Earthshock and The Caves of Androzani, that’s impressive.

So, as I’ve mulled over this one, I’ve been surprised to find myself returning to something much more contemplative about this noisy, nervy tale: about what it says about the Doctor and his inherent contradictions.

Let me take a step back: I’ve been debating the merits of The Waters of Mars with Nathan Bottomley of the Flight Through Entirety podcast. He’s got a problem with its central premise, which hangs on one of Doctor Who’s long established rules: that you can’t rewrite history, not one line. But that rule has no basis in the real world; it’s just a made-up piece of sci-fi flim flam. So we have a Doctor Who story seeking to create drama out of a fictional conceit and that weakens the whole story, making it less compelling for the audience.

He has a point, I think. Who can blame an audience for not caring about the transgression of some obscure Whovian law. But for me, the drama of The Waters of Mars feels more important than that. As the Doctor’s curiosity about solving the mystery of what happened on Bowie Base One gives way to anguish about whether or not he should save these brave, compassionate humans, this becomes more than an argument about the console room’s state of temporal grace or the favourite colours of the Prydonian chapter. It feels like there’s more at stake than that. I think the reason why, is that the made up sci-fi laws are not as insubstantial as all that: they’re a stand in for the Doctor’s moral code.

Which brings us around to asking, “what exactly is the Doctor’s moral code?” And to answer that, we need to know who he is. And that’s difficult to answer because who he is changes. Initially, he was a scientist, an engineer and a researcher, as well as a fugitive from his people. Later, he becomes a hero and a renegade. Sometimes it’s more straightforward than that, when he’s positioned as “simply a traveller.” None of these suggest someone who sticks to too many rules. But he definitely has an authoritarian streak: he can’t abide people mucking about with time.

It’s a contradiction which has grown to mythic status; in the New Adventures range, he was called “time’s champion” but it’s more accurate, at least in the classic series view, to call him “time’s policeman”. He’s not a defender, but an enforcer. In The Time Warrior, he characterises his own people as “galactic ticket inspectors” and that’s basically the role he takes.  One of his earliest recurring enemies was specifically a time meddler, whose meddling the Doctor was intent on stamping out.

Of course, his stance on the sanctity of history is not without its own contradictions. He meddles in future history and the history of other planets all the time. The basic rule is if it was taught on the history syllabus of one the show’s writers, it was inviolable. And he himself frequently brags about his influence on history – dropping apples on Isaac Newton’s head, for instance – and he loves starting famous historical fires, be they in Rome or London.

These hypocrisies aside, I think the weight of evidence tells us that the sanctity of history is part of the Doctor’s moral code. From The Aztecs all the way through to The Fires of Pompeii, the Doctor hangs on to his lost civilisation’s rule about history, as if it were a lifeline to his own. For a universally famous rule breaker, this is the one rule clings to. It’s as important to him as treating all life with respect or that blue people have the same rights as purple people.

And he believes it like a dogma, something so ingrained into him that he struggles to explain it. Like when he says to Adelaide Brook (Lindsay Duncan), this mission’s steely commander, “Your death is fixed in time forever. And that’s right.” It’s the sort of thing you say when you can’t quite describe a complex operating principle, but you know that it’s true. It’s the sort of thing a religion might preach. Don’t question it, because I fundamentally believe it to be true, the Doctor is saying.

So that’s why it’s such a lurch when he then rejects his own rule and turns around to rescue what’s left of the crew. It’s not dramatic because he’s breaking the fictional laws of time, it’s dramatic because our hero is breaking his own moral code. That’s a conflict as old as the hills, and it’s a good one. Because it signals two things: 1. That the Doctor’s in conflict with himself and you can tell that in Tennant’s hair quivering performance. He’s forcing himself to do things which he knows are against his own personal beliefs. 2. That if the Doctor’s prepared to throw this part of this moral compass aside, then – blimey (as he himself might say) – what’s next? The sanctity of life? The commitment to peaceful co-existence?.

When the Doctor abandons his moral code, bad things happen. When ground down by acts of violence and injustice all around him, or when isolated from his human companions, he’s prone to going too far. It happens here but also in The Girl Who Died, where he makes a rash decision to create an immortal teenager and in Hell Bent, where he steals, shoots and bullies his way into rescuing his friend who should be dead. And of course, the foundation of the new series was that he destroyed his own people in a moment of war-inflicted desperation, an act so incompatible with his morals that it caused him untold anguish.

The Waters of Mars asks the question, how can these various aspects of the Doctor’s personality be squared away? In the choice between being a hero and being a rule enforcer, the Doctor has chosen enforcer time and time again, from the streets of Paris to the streets of Pompeii. Here for the first time, he chooses to be a hero, but in doing so he unleashes his inner monster. It’s that internal conflict which provides the real drama here, and we care about it because we care about the Doctor. What he stands for matters to us.

(At least until The Day of the Doctor when he decides to change history, rescue everyone and everything stays fine. And in the next story, there’s a whole plot based on Whovian lore on the regeneration limit of 12. But hey – Vive la contradiction.)

SACRIFICIAL BLAM: Poor hapless Ed, played by Australia’s own Peter O’Brien.

LINK TO Kerblam!: written by Australia’s own Pete McTighe.

NEXT TIME: So this is the great journey of life! We’re stuck on The Horns of Nimon, you meddlesome hussy!