It's become somewhat of a tradition for a Doctor Who special to air on Christmas Day, with this year marking its twelfth in a row.

The episode, titled Twice Upon A Time, will see Peter Capaldi make his last outing as the Time Lord.

He will also be joined by Harry Potter and Game of Thrones actor David Bradley - who will play the first incarnation of the Doctor - a role which was originally portrayed by William Hartnell, who died in 1975.

Fans can also look forward to a few surprise returns, as well as the highly-anticipated debut of Broadchurch star Jodie Whittaker as the first female to play the iconic role.

Twice Upon A Time will also be the last episode for head writer Steven Moffat who will bow out after working on Doctor Who for 14 years.

Here is everything you need to know about the Christmas special.

When does it air?

Twice Upon A Time will air on Christmas Day at 5.30pm, on BBC One.

It will be an hour long special, making it 15 minutes longer than a regular 45 minute episode.

What is it about?

Peter Capaldi, as the twelfth Doctor, is set to reunite with his first incarnation (David Bradley) and the pair are desperately trying to avoid regeneration.

Speaking at MCM London Comic Con, David said: “It [the episode], brings together these two Doctors, who are both going through a personal crisis about [whether they should be] regenerating, or just ending it all there.

“And they’re both on this parallel journey of doubt and fear and everything.”

The BBC says of the Christmas special: "Two Doctors stranded in an arctic snowscape, refusing to face regeneration.

"Enchanted Glass People, stealing their victims from frozen time.

"And a World War One Captain destined to die on the battlefield, but taken from the trenches to play his part in the Doctor's story.

"An uplifting new tale about the power of hope in humanity’s darkest hours, Twice Upon A Time marks the end of an era. But as the Doctor must face his past to decide his future, his journey is only just beginning..."

Who is in it?

Peter Capaldi

On his departure from the show, Peter said: "Doctor Who has taken me on an amazing journey. Now that it’s coming to an end, I wish the Doctor all the very best for the future, and the past, and everything in-between. Time I was off."

Jodie Whittaker

Jodie will appear at the end of the episode, where she will emerge right after Peter's regeneration scene.

You'll have to wait until the new series to find out more about her role as the first female Doctor.

In November, the BBC released the first pictures of the actress as the Doctor. She can be seen wearing braces and a grey anorak.

Pearl Mackie

The Doctor's departed companion Bill will make a one-off return, however Pearl has hinted that her character might not be in a good place when viewers catch up with her next. The actress said: "Bill is 100% back with the full Bill energy – but she’s not quite all she seems…”

David Bradley

Fans first got to see David in character in a special clip released for Children in Need in November. Here he was seen entering the TARDIS with bewilderment, where he was met by Peter Capaldi.

He featured again in the official trailer, when a black and white archive film of William Hartnell was distorted and morphed into colour footage of David.

The opening line “Love, pride, hate, fear. Have you no emotions, Sir?” is taken from The Tenth Planet, a Doctor Who four-part special from 1966.

Mark Gatiss

The actor, comedian and writer is most recognised as playing Mycroft in Sherlock, as well as forming comedy sketch group The League Of Gentlemen alongside Reece Shearsmith, Steve Pemberton and Jeremy Dyson.

Mark will play The Captain, a World War I soldier in the Christmas special. We first got a look at his character in a clip released for Children in Need, alongside David Bradley.

He also briefly appeared in the first official trailer when he asked Peter and David if either of them is a Doctor.

However, the actor has revealed he will not be in the upcoming series following his appearance in the Christmas Special.

Jenna Coleman

It is rumoured that the Victoria actress will make a brief appearance as Clara Oswald, but this is yet to be confirmed.