When it’s Doctor Who, usually. For a TV show where the lead character is a time traveller, there is very little regard for the implications inherent in time travel, and they’re usually presented in contradictory ways when they are mentioned.
For instance: I was watching The Shakespeare Code, wherein the Doctor explains to Martha that if they fail to foil the Carrionites’ plan, the world will be destroyed right then and there in 1599 and Martha’s world will cease to exist. This is surely in contradiction of the Doctor’s assertion in the Waters of Mars that some points in history are fixed, and must stand no matter what; if the world had ended in 1599, Adelaide Brooke and her base on Mars would surely have been wiped from history too. I realise this is probably a pedantic point, but it reaches to the heart of one of the really major failings of the Russell T. Davies (RTD) era of the show: that logic and consistency would be thrown out of the window in favour of whichever plot mechanic was found most expedient at that point. It’s an ultimately unsatisfying way of telling a story.
To some extent Doctor Who has an excuse for poor continuity, with the consistency of past events with current ones; as Paul Cornell (writer of the excellent Human Nature/Family of Blood two-parter) point out, the effects of (to a greater extent) the Time War, and (to a lesser extent) time travel can re-write history, meaning that it’s not impossible to see two totally contradictory versions of events.
This provides no explanation, however, for the inconsistency in the nature of time travel itself; one would expect the laws of physics to be invariant under any sort of temporal abuse.
I’m greatly hoping that Moffat’s era will tread more carefully in this area, and the evidence presented by his old episodes show that he’s one of the few writers on the series who treats the time-travel mechanic as much more than a convenient plot generator; his first episode, The Empty Child, has a plot in which Jack Harkness runs self-cleaning scams, where the evidence is cleared away by a known historical event. Blink employs a non-linear narrative, where the Doctor invokes non-linear causality to justify him reading aloud from a transcript of a conversation he’s still having! This is all a great step up from the usual uses of time travel, which is to enter some historical period at the start of an episode, and then leave it again at the end.
Moffat’s stories show that it’s possible to tell compelling stories which are properly guided by their own consistent internal logic, requiring no great leaps of suspension of disbelief.
I was thinking about this in relation to an idea while reading the post by Paul Cornell I linked above, and combining it with my viewing of The Shakespeare Code; if time travel is re-writing history, would there have even been a threat to Earth in Shakespeare’s time if the Doctor had never gone there? Sure, he saved the day, but is just his presence in any point of history destructive to the timeline? Monsters and chaos and death seem to follow the Doctor, but is it coincidence or causation?
I think that would put a potentially interesting spin on the nature of the Doctor and the Time Lord’s policy of non-interference if the presence of time travellers was actually harmful. It would certainly put a dent into the Doctor’s otherwise impeccable morals.
I suppose this is slightly ruled out by Turn Left, in which we see the monsters would have come anyway, but I still think it would be interesting to explore.
You can tell compelling, emotional stories without totally fucking up your internal consistency. Y’know, unlike the ones told with unbreakable time-locks, for instance. Which are breakable if you want to risk going crazy. Or you can toss a diamond into a hologram as well as Timothy Dalton. I do know a lot of people who were confused by that reveal: “I thought the time lords were dead, not trapped in a bubble!”. The de-emphasis of the mechanics of time travel in the series is, I think, one of the contributors to that lack of understanding.
I do have to give RTD some credit, though. I am desperatly curious about what exactly the Could-Have-Been King and the Nightmare Child are. I suppose it’s easier to come up with cool-sounding names when there’s no requirement to ever actually have to depict them on screen!