The Arthur Dent Paradox

Writing a show about time travel can mess with your head. It can also, if you're not careful, rob your stories of any sense of real jeopardy. If people are paying enough attention.

Let's begin with The Visitation, in which the Fifth Doctor accidentally starts the Great Fire of London. No reason why not. But consider the fact that the Fourth Doctor has heard of the Fire in Pyramids of Mars; St Paul's Dome is visible in The Invasion; and Monument Tube Station appears in The Web of Fear. So, the Great Fire had already happened, before the time traveller made it happen.

It's even more explicit in new season 3. The Master regenerates in Utopia. He speaks his first words and Martha, overhearing them, recognises his voice. He then goes back in time and becomes Harold Saxon and runs for office: which Martha and Jack knew all about before Professor Yana is woken up.

This does rather suggest a form of predestination. The Family of Blood couldn't kill the Doctor in episode 9 of season 3 because the effects of his actions in episode 11 had already been shown in episode 6. Indeed, once the 2nd Doctor knows about the Monument, he's presumably safe until The Visitation. Farewell jeopardy.

I don't know if this has a name, but I'm going to call it the Arthur Dent paradox. In Life, The Universe and Everything (book 3 of Hitch Hiker's), Arthur learns that one day, he will visit Stavromula Beta and accidentally cause the death of a man called Agrajag. Throughout the next two books, he is calmly convinced that he can't die until he has visited Stavromula Beta. It's only logical and, what's more, true: when he does (apparently) die at the end of Mostly Harmless, it's shortly after the Stavromula Beta experience, which is not quite what he'd imagined.

This has caused problems. When writing The End of Time, Russell T Davies realised that, thanks to a gag at the end of The Shakespeare Code, the Tenth Doctor had to meet and offend Elizabeth I before he could regenerate. Hence a few lined crowbarred in and unfortunately referenced more than once since.

More seriously, the plot of The Impossible Astronaut required the audience to believe that Amy, Rory and River believed the Doctor was dead. But we know River has a diary of all her encounters with him, so she'd know this can't be the end. Which meant Moffat had to write a scene making it clear that they've done everything now, including Jim the Fish.

But in that case, clearly River's entire relationship with the Doctor is with the Matt Smith version. And she seems to have only met another incarnation once: on the day she died. Strange how she didn't remark on this.

Yeah, it messes with your head. But, like Arthur Dent, it also makes the Doctor invincible. He was perfectly safe from acid rain at Christmas because he hasn't visited the fields of Trenzalore! Thanks to Dorium's future knowledge, his safety is assured.

But then he was already safe. He hasn't been Merlin yet!