Everything you need to know about classic Doctor Who story the Tenth Planet

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However, it’s likely that you haven’t seen this story on TV before as A) it was first broadcast in 1966, and B) the last quarter of the four-part series has been lost for decades and only exists in audio/animated form.

Fortunately though, we’ve materialised a quick-fire Q&A that should douse any burning questions you have about the meeting of classic and modern Who...

What episode does The First Doctor deliver the line “Love, pride, hate, fear. Have you no emotions, sir?”

You know the one: it’s from the opening scene of the Twice Upon a Time trailer – the sequence that morphs between archive film of Hartnell and new colour footage of Bradley.

The line is taken from The Tenth Planet, a Doctor Who four-part serial from 1966 (as discussed above). It featured Hartnell as The First Doctor, his last story as the Tardis’ incumbent Time Lord.

What happens in The Tenth Planet?

The story starts with The Doctor and his companions of the time – Polly and Ben – materialising at Snowcap, a space tracking station based at Earth’s South Pole. The year is 1986, 20 years in the future to a 1966 audience.

However, the time-travelling trio don't have much time to enjoy this futuristic polar base: as ever, The Doctor soon discovers a massive problem. An unknown planet has joined the solar system (making it the tenth planet to orbit the sun* – hence the title) and is hurtling towards Earth. DUM DUM DUMM.

And things suddenly get a lot worse when the tracking station is swarmed by strange metal robo-monsters: the Cybermen, in their first ever screen appearance. However, these aren’t the strong steel cyberman from the Matt Smith/David Tennant era, but the basic white balaclava beings we saw at the end of series 10.

Don’t worry: however scary their appearance and worrying their speech inflexions, these Cyberman looked very silly karate chopping their enemies...

After taking over Snowcap, the Cybermen reveal they've rocketed to Earth from the inbound tenth planet, their homeworld Mondas. And it turns out they have an evil scheme: convert all humans into Cyberman before sucking up all energy from Earth. (This plan also involves a bit about Z-bombs, Geneva and a spaceship called Zeus IV, but let’s keep this simple, eh?).

For a while, the Cyber plan goes smoothly: Mondas begins to absorb Earth’s power and the Cybermen take The Doctor and Polly back to their Cybership as prisoners. However, they fail to realise that Mondas is absorbing too much energy and the planet explodes, shutting down all Cybermen.

After the monsters are defeated, Ben rushes to free Polly and the semi-conscious Doctor from their Cybercages, telling the Time Lord “it's all over now”. The weary Doctor doesn’t agree, however, replying, “but it isn't all over. It's far from being all over..."

The Doctor then demands to get back to his Tardis immediately and leaves the Cybership for the polar landscape without his companions.

According to the BBC, before returning to his time machine, The Doctor was also supposed to deliver the line "No, I can't go through with it! I can't. I will not give in!". The line was cut from the final episode, but it indicates that, just like his twelfth incarnation, The First Doctor was trying to fight his regeneration.

So, is that where Twice Upon a Time fits in?

Yes. According to Doctor Who magazine, the events in the Christmas special occur in the period between The Doctor leaving the Cybership and returning to his Tardis at the end of Tenth Planet.

Of course, we’ve already seen where the Twelfth Doctor's story and The Tenth Planet intersects: remember when Capaldi's Doctor comes across Bradley's at the end of series 10 finale The Doctor Falls? That's when The First Doctor is returning to his Tardis.

Interesting, so what happens at the end of the Tenth Planet? Does the First Doctor regenerate?

Correct! In the final scene, we see Polly and Ben arriving at the Tardis where the First Doctor collapses on the floor with a mysterious bright light beaming from his face which changes, becoming younger. He now has the body of Patrick Troughton, the Second Doctor.

However, this wasn’t called a ‘regeneration’ back in 1966 – that word wasn't used until eight years and two Doctors later in 1974’s Planet of the Spiders. In fact, at the time of The Tenth Planet’s first broadcast, Who fans didn’t even know The Doctor was a Time Lord – this was was only revealed during 1969 story The War Games.

Wasn’t William Hartnell ill during filming?

Great Who Knowledge: the man who played The First Doctor was indeed in poor health during his last Who story and had to be written out of part of The Tenth Planet. As Hartnell was too ill to appear in the third episode of the story, writer Gerry Davis re-worked the script.

This re-write saw The Doctor mysteriously fall unconscious at the start of episode three and his lines were handed out to other characters, particularly Ben.

You said earlier some of The Tenth Planet footage was lost?

Yes, as is the case with many of the classic Doctor Who stories, part of The Tenth Planet is missing, with the last episode reduced to audio with stills and scene directions. And since William Hartnell was too ill to feature in the third episode, this means the last surviving First Doctor story doesn’t actually feature the First Doctor.

However, just like lost Tom Baker-era serial Shada, the Who team have made an animated reconstruction of the fourth episode of The Tenth Planet, available on the special DVD release of the episodes.

Hang on, if the last episode of The Tenth Planet is lost, how do we still have footage of William Hartnell’s regeneration?

Like so many things, it’s all thanks to Blue Peter. A copy of the sequence was used for a special Who edition of the kid’s show, which discussed The First Doctor’s regeneration. And unlike The Tenth Planet itself, this Blue Peter episode hasn’t gone missing in space and time.

This episode of Blue Peter inadvertently also preserved other key Doctor Who sequences, including Katarina's death in The Daleks' Master Plan.

*Of course, today’, a new planet would be our solar system’s ninth as, since 2006, Pluto has not been classified as a planet.

Doctor Who: Twice Upon a Time airs on BBC1 on Christmas Day (Monday 25th December) at 5:30pm