8 Years Ago, Doctor Who Nailed a Time-Travel Trope That Loki is Just Starting to Use

Get your bootstraps on!

Peter Capaldi with a guitar as The Doctor in 'Doctor Who.'
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Today, whenever sci-fi fans are talking enthusiastically about time paradoxes in time-travel storylines, it's tempting to evoke the immortal words of the 10th Doctor (David Tennant) and say: “Wibbly Wobbly Timey Wimey.” This phrase comes from the famous episode “Blink,” in which predestination paradoxes save the day. But, arguably, the better, and more mind-blowing Doctor Who paradox happened in 2015 with the two-part story “Under the Lake” and “Before the Flood.” Eight years ago, on October 10, 2015, Doctor Who threw down with one of the best bootstraps paradoxes of all time, giving other sci-fi franchises — like Loki Season 2 — a near-perfect model of how to make sense of the infinite loop of the bootstrap paradox. Spoilers ahead.

In the opening moments of “Before the Flood,” the 12th Doctor (Peter Capaldi) gives a now-classic illustrative example of how a bootstrap paradox works, and why it’s logistically confounding. In a very rare breaking of the fourth wall, the Doctor relates a hilarious (and untrue) account of a time traveler who is such a fan of Ludwig van Beethoven that he goes back in time to meet Beethoven, only to discover Beethoven doesn’t exist.

In the Doctor’s story, the hypothetical time traveler becomes Beethoven, because he happens to have all of Beethoven’s sheet music from the future. But the question is, as the Doctor says, “Who composed Beethoven’s 5th?”

Sometimes called an ontological paradox, the bootstrap paradox is the moment when the result of time travel creates a thing, a person, or an object in which the first origin of said thing is unclear. With all the pruning of timelines in Loki, and the god of mischief pulling himself out of various timelines, the implications of the bootstrap paradox can get exciting when you have various realities overlapping and an entire Marvel Cinematic Universe in chaos. But, what makes Doctor Who’s approach great is in its smallness. When time paradoxes start to have multiverse-altering impacts, the stakes, arguably, can get a little muddled. But in “Before the Flood,” Doctor Who centers its stakes around how information is birthed through paradoxes.

Written by Toby Whithouse, the two-part episode largely involves an alien invader called The Fisher King and an underwater base that is overrun with ghosts. Worse still, the ghosts can convert their victims into ghosts, which is part of a complex invasion plan. At some point, the Doctor’s ghost appears from the future, which makes the audience think the Doctor will lose at some point. However, it’s all a holographic ruse, so the Doctor can give himself and Clara a specific message. But, how did the Doctor know what message to send via his hologram ghost? The answer is time travel. Which, creates the bigger question: Where did the message come from in the first place?

The Doctor (Peter Capaldi) gets ready to make a paradox.


In “Under the Lake” and “Before the Flood,” the Doctor wins the day through a battle of information and information paradoxes. He defeats the Fisher King through misdirection, but also because he has information the Fisher King doesn’t. But, the true ace of the Doctor’s sleeve is the bootstrap paradox itself. This wasn’t the first time Doctor Who leaned on this idea in the Steven Moffat-era of the series — in the 2010 Matt Smith episode “The Big Bang,” the Doctor gets himself out of the locked box of the Pandorica because he puts his sonic screwdriver in Amy Pond’s pocket after he’s gotten himself out with said sonic.

Nothing about this makes sense from a linear cause-and-effect point of view, which is why the bootstrap paradox is such a compelling — and often hilarious — time travel trope. What’s interesting about “Before the Flood,” is that Doctor Who is a time-travel show that often seems to break its own rules about time travel. In this episode, the concept of how the paradox “works” is laid out in the Doctor’s monologue, and then, the rest of the episode demonstrates how the Doctor uses that paradox to save the day. Now, if Doctor Who did this kind of thing with every single episode, it would get old, which is why, curiously, the famous time-travel series is often not centered on time paradoxes. Seriously, you can probably count the number of episodes from the pre-2005 Who era that are actually about time paradoxes on one hand.

Loki Season 2’s bootstrap paradoxes only scratch the surface of Doctor Who’s paradoxes.

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Other time travel sci-fi series — from 12 Monkeys to Loki — have employed bootstrap paradoxes in all sorts of unique ways. But, if you’re looking for a masterclass in mind-bending plot twists, combined with down-to-earth stakes, this modern classic of Doctor Who just keeps getting better, year after year. Unless of course, Toby Whithouse just read this article and is about to pop back in the TARDIS to the year 2015.

Doctor Who currently streams on Max. The contemporary series will air on Disney+ at some point in November 2023 with the debut of the 60th anniversary specials.

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