They have never had the most straightforward relationship. But now Amy Pond – childhood friend, mother-in-law and long-serving companion – is to wave goodbye to the Doctor as she bows out of Doctor Who.

Pond, played by Karen Gillan, will leave the Saturday teatime family show during the next series – but not until she's been reunited with the Doctor.

"Amy and [husband] Rory will be rejoining us next series and joining the Doctor back on the Tardis," Doctor Who showrunner Steven Moffat confirmed after a screening of this year's Doctor Who Christmas Day special.

"But the final days of the Ponds are coming – and during the next series. I'm not telling you when and certainly not telling you how. But that story is going to come to a heartbreaking end."

This year's festive Doctor Who is itself stuffed with heartbreaking moments, as we spend Christmas with an evacuated family in the second world war. But the departure of strong-willed Amy will be mourned by many fans.

"There are only so many more adventures with the lovely Amy and the lovely Rory. So that story, next series, during the series will be over," Moffat said. "And then the Doctor is going to meet a new friend – and I'm not going to tell you anything about he, she or it!"

Karen Gillan's character first arrived on screen dressed as a kissogram during Matt Smith's first episode as the Doctor. "We've had the most incredible journey," said Smith of Gillan.

"We took over the show and we really had to hold hands and help each other through it in many ways. So it's very disappointing, but one has to remember that this show is about change and regeneration and that's what galvanises it and moves it forward. It's about looking forward, always."

A new companion for the Doctor has not yet been cast – with Moffat even teasing that it could be an alien. "Who knows? It is Doctor Who. There's a universe of people to choose from," he said.

This year's Christmas special contains a not too subtle nod to The Chronicles of Narnia in its title – and it won't take long for fans to spot the debt The Doctor, the Widow and the Wardrobe owes to CS Lewis .

Outnumbered's Claire Skinner is zipped back in time to play a mum in second-world-war England looking after her two evacuated children with the help – supposedly – of a rather overexcited Doctor.

Bill Bailey and Arabella Weir make charming – and hilarious – appearances later in the episode.

As is traditional, the show, which airs at 7pm on Christmas Day, is filled with tear-jerking moments – even if it takes the Doctor a while to get his head round what exactly all the weeping is for.

"I think Doctor Who always [wears its] hearts on its sleeves – but yes, Christmas Day is a big, sentimental, loving day and the Doctor on Christmas Day [is] just a little bit more Willy Wonka somehow," said Moffat.

"It's the kind of day that's made for him: nobody minds being silly, no one worries about good taste. People just like glittery things and chocolate for breakfast."

An excited audience of fans included a surprising number of politicians with their children, including the chancellor, shadow chancellor, Charles Kennedy and Labour MP Tom Watson among others.

For the first time in more than 40 years, clips from two lost Doctor Who episodes unearthed at a village fete were shown earlier this week – with children catching perhaps their first glance of William Hartnell and Patrick Troughton as the Doctor.

Smith revealed that Patrick Troughton was his favourite Doctor, which is one possible explanation as to why the second and 11th Doctor share a love of bow ties.

Gillan is to play Jean Shrimpton next year in a BBC4 drama about the 1960s model's love affair with photographer David Bailey.

• This article was amended on 16 December 2011. The original referred to two lost Doctor Who episodes unearthed earlier this week at a village fete. This has been corrected.