Thanks to a now deleted listing on BBC America online store to pre-order the 2017 Doctor Who Christmas special, the first full synopsis for Twice Upon A Time has been revealed.

"Two Doctors stranded in a forbidding snowscape, refusing to face regeneration. And a British army captain seemingly destined to die in the First World War, but taken from the trenches to play his part in the Doctor's story.

This is the magical last chapter in the Twelfth Doctor's epic adventure. He must face his past to decide his future. And the Doctor will realise the resilience of humanity, discovering hope in his darkest frozen moment.It's the end of an era. But the Doctor's journey is only just beginning."

While that doesn't give us much more information than what was in the trailer, it does confirm (in a roundabout way) that Capaldi's finale will also serve to fill in the gap of William Hartnell's last adventure. As hardcore Whovians will know, Hartnell's final serial (The Tenth Planet) was the first story to introduce the Cybermen and features the first-ever regeneration.

However, owing to a plethora of errors from the BBC, the story remains incomplete as the final episode is still missing. Fragments of the episode including the regeneration to Patrick Troughton exist online, but the low-quality footage is all that remains of the original filmed episode. For the DVD release back in 2013, Planet 55 Studios created an animated version of the fourth episode to offer fans their first chance to see the full story.

Twice Upon A Time is truly an end of an era as head writer Steven Moffat will hand over the keys to the TARDIS to new showrunner Chris Chibnall. The highly anticipated special will also feature the debut of Jodie Whittaker as The Doctor ahead of her first full series next year.

Twice Upon A Time will be broadcast on Christmas Day on BBC One, US fans can catch it on BBC America.