There was a whole lot more to tonight's Doctor Who than we expected – at first glance, 'Thin Ice' appeared to be a fun romp with a giant monster, but like the frozen river Thames, there was something rather extraordinary lurking underneath the surface.
Join us now as we take a deep dive into the episode, asking some BIG questions about the Doctor and Bill's latest adventure.
1. What's in the Vault?
By the way Nardole (Matt Lucas) reacts, it appears to be someone rather than something inside the mysterious Vault which the Doctor has pledged to protect.
"As long as I'm still here, you are going nowhere!" Nardole vows. (It's also possible, of course, that there's more than one living being locked inside – like, say, an army of Daleks?)
Given that whoever's inside the Vault tapped 1-2-3 on its door, we can probably rule out The Master (John Simm), who – lest we forget – always knocks four times.
2. Who is Nardole working for?
We speculated last week that Nardole might have been assigned to watch over the Vault by some higher power. He certainly seems more concerned with the Doctor's "oath" than the Time Lord himself does, and it would explain why he got left behind last week.
Then again, the way in which Nardole addresses our hero as "sir" does seem to suggest that his loyalty lies with the Doctor, so maybe we're over-thinking things and he's just a bit of an irritating busybody.
3. An elephant on a frozen river Thames – that's ridiculous, right?
Nope, it actually happened. Well, probably.
Believe it or not, about 200 years ago, temperatures in London did plummet so severely that the Thames actually froze solid. The frozen river became a tourist attraction with merchants even setting up stalls on the ice.
And yes, at one stage, an elephant was led across the frozen waters, near Blackfriars Bridge, as a stunt for onlookers. So there ya go.
4. When did the Doctor visit the Frost Fair before?
The Doctor mentions to Bill in 'Thin Ice' that he's already been to the Frost Fair "a couple of times" – and indeed, 2011 episode 'A Good Man Goes to War' reveals that the eleventh Doctor and River Song visited the same festivities in 1814, to celebrate River's birthday with a little help from Stevie Wonder. No, really.
The twelfth Doctor had previously promised to take Clara to the Frost Fair in 2014's 'The Caretaker', and eventually did, at least according to the spin-off book Silhouette.
If you want to get super-nerdy (and we do), the first Doctor also visited the last great Frost Fair in the Big Finish audio Frostfire, as did the tenth Doctor in another book, The Frozen, while the second Doctor arrived at a different fair – in 1648 – in the novel The Roundheads.
5. Did Doctor Who just confirm the existence of Jesus?
"Bit more black than they show in the movies," Bill notes of Regency London. "So was Jesus," the Doctor replies. "History's a whitewash."
Should we take this as confirmation that the Doctor has met Jesus, who very much exists as a character in the Whoniverse?
Back in the early '60s, when the series was but a gleam in the eye of writer C.E. Webber, he suggested that the Doctor should witness the birth of Christ as part of the show's first Christmas episode. Maybe posing as one of the Three Wise Men?
6. Can poop really be used as fuel?
Oh yeah, we only tackle the really important issues here on Digital Spy.
Lord Sutcliffe's plot in 'Thin Ice' hinges on feeding tourists from the Frost Fair to a monster hidden beneath the ice, with the waste it produces being used as a phenomenally powerful space fuel.
In fact, NASA has actually hired scientists to look into whether the waste used by astronauts could likewise be turned into rocket fuel. "Human waste has been a hassle for spaceflight since the dawn of the space age – but now it could transform our push toward the final frontier," wrote tech and science website Seeker in 2014.
Every day's a school day.
7. Can the TARDIS travel to parallel worlds?
Bill assumes that she and the Doctor have landed on a parallel world when they glimpse an elephant on a frozen Thames. In fact, she's just arrived in a particularly outlandish point in our Earth's history.
But that's not to say the TARDIS can't travel to alternate universes. The third Doctor accidentally transported himself to a parallel Earth where the UK was a fascist Republic in 1970's 'Inferno', while the idea of an alternate universe was pivotal to series two of the revived series – the tenth Doctor (David Tennant) battled Cyberman and Daleks on another Earth, where his beloved Rose Tyler (Billie Piper) ultimately ended up trapped.
And, of course, Gallifrey was briefly hidden in a pocket universe to save the Time Lords from the Daleks at the end of the last great Time War.
8. Are the Doctor and Bill parting ways?
Each episode of series 10 has so far led directly into the next, but judging from the trailer for next week's outing, there's a gap between 'Thin Ice' and 'Knock, Knock' – a big one.
Bill's apparently been on Earth long enough to have found a new living space, a grand but rather spooky-looking old house she's sharing with five friends. Will she be a "part-time" companion from now, jumping on and off the TARDIS as it suits, like Clara before her?
9. Why does no-one in Doctor Who use Google?
Bill uses "Search-Wise" to look into the historical consequences of her trip to the past – the same search engine that Rose used to look up "doctor blue box" back in 'Rose' (2005).
Though it's not a real search engine, Search-Wise.net is a real website you can visit, designed for use in films and TV shows that don't want to give Google free advertising. Not that it needs it.
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