In Terry Pratchett's Johnny and the Bomb, chapter 9, Kirsty tries to explain that Wobbler couldn't come back to the future with his friends because he changed history and accidentally killed his own grandfather.

Wobbler gave them a suspicious look.

‘What's going on here?’ he said.

‘Look, I can't explain now,’ said Johnny. ‘You're … stuck here. Er. Apparently, er, something's gone wrong. Er. There's been a snag.’

‘What kind of snag?’

‘Er. Quite a big one.’

Wobbler stopped eating. It was that serious.

‘How big?’ he said.

‘Er. You're not going to be born … er.’ Wobbler stared at him. Then he stared at the half-eaten burger.

‘Am I eating this burger? Are these my teeth marks?’ he demanded.

‘Look, it's perfectly simple,’ said Kirsty. ‘You're alive here, yes, but when we first came back, something must have happened which changed history. Everything anyone does changes history. So there's two histories. You were born in one, but things have been changed and when we got back it was into a different history where you weren't. All we have to do is put things back the way they should be, and then everything will be all right.’

‘Hah! You haven't got a shelf of Star Trek videos as well, have you?’ said Wobbler.

Was Wobbler thinking of a particular episode (or film or other story) of Star Trek here, one that has similar trouble with time travel and the dangers of changing the past?

Please consider only episodes of which both Wobbler (a kid in the UK) could have known of in 1996 (when he lived at the time of this story), and Pratchett could have known of in 1996 (when Johnny and the Bombs was first published).

The dialog about time travel in chapter 1, which foreshadows much of the plot, may also be relevant:

‘But you can't travel in time without messing things up,’ said Yo-less.

‘That's the whole point,’ said Bigmac. ‘That's what you want to do. I wouldn't mind joining the police if they were time police. You'd go back and say, “Hey, are you Adolf Hitler?” and when he said, “Achtung, that's me, ja” … Kablooeee! With the pumb-action shotgun. End of problem.’

‘Yes, but supposing you accidentally shot your own grandfather,’ said Yo-less patiently.

‘I wouldn't. He doesn't look a bit like Adolf Hitler.’

‘Anyway, you're not that good a shot,’ said Wobbler. [skipping the anecdote about Bigmac]

‘I didn't mean actually shooting your actual grandfather,’ said Yo-less, loudly. ‘I mean messing things up so maybe you're not actually born or your time machine never gets invented. Like in that film where the robot is sent back to kill the mother of the boy who's going to beat the robots when he grows up.’¹

‘Good one, that,’ said Bigmac, strafing the silent shops with an invisible machine gun.

‘But if he never got born how did they know he'd existed?’ said Yo-less. ‘Didn't make any sense to me.’

‘How come you're such an expert?’ said Wobbler.

‘Well, I've got three shelves of Star Trek videos,’ said Yo-less.

‘Anorak alert!’



‘Anyway,’ said Yo-less, ‘if you changed things, maybe you'd end up not going back in time, and there you would be, back in time, I mean, except you never went in the first place, so you wouldn't be able to come back on account of not having gone. Or, even if you could get back, you'd get back to another time, like a sort of parallel dimension, because if the thing you changed hadn't happened then you wouldn't've gone, so you could only come back to somewhere you never went. And there you'd be – stuck.’

They tried to work this out.

‘Huh, you'd have to be mad to even understand time travel,’ said Wobbler eventually.

¹ The Annotated Pratchett File tells which film Yo-less mentions in this line.

  • 6
    From the phrasing, I don't think it's referencing Star Trek at all - Kirsty has just given a description of a common trope in science fiction, and Wobbler is using Star Trek as a catch-all for sci-fi nerdiness in general.
    – Werrf
    Apr 19, 2017 at 15:36
  • 11
    Obvious choice: CIty on the Edge of Forever.
    – Hellion
    Apr 19, 2017 at 15:37
  • 2
    "Discworld is based on a slew of old myths, which reach their most 'refined' form in Hindu mythology, which in turn of course derived from the original Star Trek episode 'Planet of Wobbly Rocks where the Security Guard Got Shot".
    – Valorum
    Apr 19, 2017 at 15:53
  • 1
    @Valorum That's great, but the Johnny Maxwell trilogy isn't part of Discworld.
    – b_jonas
    Apr 19, 2017 at 15:54
  • 2
    @Valorum Wait, why? Wouldn't TNG episodes be more likely to be available on video for young nerds in 1996?
    – b_jonas
    Apr 19, 2017 at 21:23

3 Answers 3


Given that we're talking about Star Trek tapes (meaning non-canon could apply here), there was an Animated Series episode called Yesteryear from 1973

As the travelers begin to appear, one-by-one, Kirk announces to McCoy how exciting it was to observe the birth of a civilization, just as the last traveler, Spock, emerges from the Guardian, much to the surprise of both Grey and McCoy. McCoy inquires who it is they brought back with them. Although Kirk reacts with puzzlement because he believes McCoy obviously knows Spock, McCoy says he doesn't.

Spock then realizes he's only alive because of a paradox he created himself

Spock then recalls the date of the event in question, the twentieth day of Tasmeen, noting that it was the day that his cousin Selek saved his life in the desert when he was attacked by a wild animal. Although Spock cannot remember the finer details of the event, he recalls that that was also the only time he had ever met Selek.

On a hunch, Kirk asks if Selek looked anything like how Spock now does. Spock confirms what Kirk has been thinking; Spock saved his own life in the original timeline, but was unable to do it a second time because he was in Orion's past when the time vortex replayed Vulcan history, making it impossible for him to be in two places at one time. With the realization of what has transpired, Kirk asks the Guardian if there is any way they can reverse what happened. The Guardian confirms that there is, as long as no other important factor is altered.


Could be "Tomorrow is Yesterday" - TOS Season 1: Ep 19

The crew travels to the past, and John Christopher, a US Pilot from the 1960's who accidentally sees the Enterprise, is brought on board. The crew intends to return to the future with him, but decide they can't, because his son (who is very important in terms of the timeline) hasn't been born yet:

SPOCK: Poor choice of words on my part. I neglected, in my initial run-through, to correlate the possible contributions by offspring. I find, after running a crosscheck on that factor, that your son Colonel Shaun Geoffrey Christopher headed, or will head, the first successful Earth-Saturn probe, which is a rather significant

CHRISTOPHER: Wait a minute. I don't have a son.

MCCOY: You mean yet.

SPOCK: The doctor is correct. Unless we return Captain Christopher to Earth, There will be no Colonel Shaun Geoffrey Christopher to go to Saturn.

KIRK: Well, that's it, isn't it? We'll have to find some way of

CHRISTOPHER: A boy. I'm going to have a son.

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Source: http://www.chakoteya.net/StarTrek/21.htm


The tape in question could be a reference to "City on the Edge of Forever", in which Kirk, Spock, and McCoy, at the first discovery of the Guardian of Forever, are transported back to Earth, ca. 1933, and Kirk saves the life of Edith Keeler (and falls in love with her, of course) -- before Spock discovers that they have somehow, inadvertently, wiped history of the Federation in its entirety, and traces the change to Edith Keeler living beyond the time of their arrival in the past.

This is "grandfather paradox" writ large -- the simple act of keeping a stranger from being run down by a car changes three centuries of history into an unrecognizable form.

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