Amid all the smiley faces, terror and gleaming white architecture of this week’s Doctor Who, eagle-eyed fans may have spotted Peter Capaldi’s Time Lord make a subtle allusion to an adventure from Matt Smith’s era at the helm of the Tardis – as well as an older 1970s episode starring Tom Baker.

Of course, in a series with so much history there’s usually more than just ONE episode referenced – for example, the Doctor makes a crack about Bill not reading his browser history that calls back to 2015 episode The Zygon Inversion – but the main one we’re thinking of has some particular resonances, as it was also a second episode in a series at a time when head writer Steven Moffat was trying to make everything feel fresh again.

The callback comes when new companion Bill (Pearl Mackie) discovers that the settlers on this dangerous colony left Earth under a bit of a cloud, with humanity’s home planet apparently uninhabitable due to war and environmental catastrophe.

“Doctor, why did people come here? Did something terrible happen?” Bill asks the Doctor.

“I’ve got to know – the people who came here. Were they the last people? Were they our last hope?”

The Doctor’s reply? “Earth was evacuated. But there were a number of ships. I’ve bumped into a few of them over the years.”

Now, unless Earth has been evacuated and repopulated a few times over the millennia, we’d say one of the times the Doctor is referring to must have been in 2010 episode The Beast Below, which saw Smith’s Eleventh incarnation of the Time Lord exploring a spaceship that had carried the entire UK away from a sun flare-ravaged Earth (with a little help from a captured space whale, of course).

Bill (Pearl Mackie) and Amy (Karen Gillan) both shed a tear after watching a video montage of humanity's fate

Notably, that episode featured a similar sped-up montage video of humanity’s downfall to the one watched by Bill in Smile (complete with riot police, explosions and the watcher shedding a tear, as you can see above), though the connections don’t stop there – The Beast Below also made a then-topical reference to Scottish independence, with Karen Gillan’s Amy Pond questioning their absence from Starship UK.

Mandy: You sound Scottish.

Amy: I am Scottish. What's wrong with that? Scotland's got to be here somewhere.

Mandy: No. They wanted their own ship.

Amy: Hmm. Good for them. Nothing changes.

In of itself this isn’t that notable (except that it totally predicted Brexit, and we’ll brook no arguments on the matter), but it does match up quite cleanly with a similar joke made by Capaldi’s Doctor in this new episode, cementing the fact that in the Doctor Who universe the SNP has influence spanning millennia. 

Check it his exchange with Bill below:

Bill: Why are you Scottish?

The Doctor: I’m not Scottish, I’m just cross.

Bill: Is there Scotland in space?

The Doctor: They’re all over the place. Demanding independence from every planet that they land on.

Nicola Sturgeon will be pleased.

Anyway, all this boils down to the fact that Smile seems to be hinting that the Beast Below timeline also resulted in the events of THIS episode in a roundabout way, though possibly at a much later date and with no real crossover for the events of that episode.

The Doctor's comments that he's bumped into a "couple" of such ships also seems to hint that classic 1975 Who serial The Ark in Space could exist in the same timeline, with the Tom Baker story featuring a similar set-up of colonists kept in suspended animation.

Overall, it’s just a neat little nod for observant fans that nicely connects different strands of humanity’s fictional history in Doctor Who and probably doesn’t warrant this much dissection and explanation. But you can hardly blame us for getting a bit excited – after all, we’re only trying to make you smile.

Doctor Who continues on BBC1 on Saturdays at 7.20pm