Where did we leave things at the end of the last episode of Doctor Who?

Fortunately, we’ve compiled a cheat sheet so you’re up to date with your Doctor Who knowledge in time (and space) for The Return of Doctor Mysterio.

Where did we leave River Song?

On Christmas Day 5343, the Doctor encountered his former wife on the human colony of Mendorax Dellora. She didn't initially recognise him as it was the first time she'd seen him in his Twelfth Doctor incarnation. But she enlisted his help anyway to try to steal the most valuable diamond in the universe – the Halassi Androvar – which had become embedded in the head of cybernetic warlord King Hydroflax (the details behind that are unimportant).

As Doctor Who fans will know, River and the Doctor's story is both a head-scratcher and a tear-jerker because the first time they meet, during the Silence in the Library/Forest of the Dead story arc, it's the last time she sees him: River ends up sacrificing her own life to save The Doctor. So when the Twelfth Doctor discovers during the Husbands of River Song that the next stop in her timeline is that very library, it's a pretty heartbreaking moment.

They share a last special experience together – a date in a restaurant with a view of the famous Singing Towers of Darillium, where the nights last for 24 years – so there's a good chance that when we next see the Doctor in The Return of Doctor Mysterio he'll still be grieving over that last, long farewell.

Who is Nardole?

Played by Matt Lucas, Nardole is the nice-but-dim human servant of River Song.

Not far into last year's festive special, Nardole is decapitated by the aforementioned King Hydroflax. But it’s not as bloody as it sounds: Nardole’s head is quickly attached to Hydroflax's robot body, preventing Nardole from dying (and any blood stains on the carpet).

Luckily for Nardole, Hydroflax is soon defeated and Nardole gains control of the cyber-bod. Unfortunately, he has to share that control with Ramone, a man River Song married in order to help her steal the aforementioned diamond.

Cyborg Nadole in the 2015 Christmas special

And that’s the last time we see Nardole until this year’s Christmas special in which he appears to be back his own body. How and why we’ll just have to wait to find out.

Nardole in the 2016 Christmas special

Who are the Shoal of the Winter Harmony?

They’re villains from last year’s special who have a neat little party trick: they can easily get off their faces. No, literally: they can tear their heads apart to reveal a blended Avatar substance inside.

So far, so gross. But the Shoal also have the ability to take over any body with their blue gunk and in The Husbands of River Song, the body they want is The Doctor’s. A nice objective, but it doesn’t take into account the meteor storm that soon cripples their ship – d’oh!

However, despite their misfortune last year, there’s a good chance the Shoal will be back in this year’s Christmas special. Not only has Steve Moffat promised the monsters will be “something we’ve seen before but quite recently”, but eagle-eyed fans have also spotted the name “Harmony Shoal” in a preview clip for The Return of Doctor Mysterio.

Also, remember the Shoal’s head-splitting ability? Well, it leaves a scar across their face that even a Bond villain would describe as ‘a tad showy’. And we can see such a scar on one of the characters in the same preview clip.

So that's your briefing on some of the key elements from the last episode of Doctor Who that could impact on the next one, this year's Christmas special.

Now go and treat yourself to a mince pie and a glass of sherry while you count down the hours until The Return of Doctor Mysterio...

The Doctor Who Christmas special, The Return of Doctor Mysterio, is at 5:45pm on BBC1 on Christmas Day