In this year’s Call The Midwife Christmas special, the episode opens in 2005, with Jennifer Worth (Vanessa Redgrave) reminiscing about life as a young midwife during the 1950s. The hit drama, which returns for a fourth series in January, was inspired by Jennifer’s memoirs, but she passed away in 2011, before Call The Midwife made it on the air.

From 2005, we’re transported back to 1959, where the midwives and sisters of Nonnatus House are busily preparing for the hectic festive season. As well as treating east London’s infants and mothers-to-be, the midwives are helping with rehearsals for the Sunday School Christmas Concert.

“The Christmas special is as delightfully Christmassy as ever,” says Miranda Hart, who plays Camilla “Chummy” Noakes. “Chummy puts a show together with little girls playing snowflakes – it’s very sweet.”

But it’s not all mince pies and merriment for the team…

“As always, things are a little bit complicated,” explains Emerald Fennell, 29, who joined the cast as junior midwife Patsy Mount in the last series.

“Patsy and Chummy find themselves looking after a home for pregnant, unmarried young women.”

Being an unmarried mother was still a taboo at the time and many women were forced, often against their will, to go to these homes to give birth before giving their babies away for adoption.

“Very young, unmarried girls ended up there because they were pregnant. They care for their babies over six weeks, which is enough time to have a real bond with the child, then they have to hand them over,” Miranda tells the Daily Express.

“Sister Maltby [Ruth Sheen] runs the home and is meant to be a very kind, caring woman, but she isn’t. These girls are not being cared for emotionally or physically, so it’s quite shocking.

“It’s Chummy to the rescue, I say – ticketyboo!”

Elsewhere, Cynthia (Bryony Hannah) finds herself drawn into a case concerning two former residents of a Victorian mental hospital, Victor (Daniel Cerqueira) and Nancy (former EastEnders star Kacey Ainsworth). Their story has a profound effect on Cynthia as she struggles with a major life decision.

“The brilliant thing about Call The Midwife is that you’ve got your mince pies and your snowflakes and reindeer, but then you’ve also got these incredibly harrowing stories that will touch people in the audience,” says Bryony. “And that’s what Christmas is about, isn’t it? Laughter and tears.”

As Cynthia contemplates her life, her glamorous colleague Trixie (Helen George) is also pondering her future with local vicar Tom Hereward (Jack Ashton).

“Trixie is really enjoying Christmas with the vicar as they’ve been dating a while now, but there’s an underlying current of Cynthia’s big decision which will affect their friendship.”

“The Christmas episode weaves all the main characters throughout the stories. We are allowed a little more time in this episode, so the relationships and storylines can be looked at in depth,” adds Jenny Agutter, who plays Sister Julienne.

“It sets a great deal up for the coming series – it’s the end of the Fifties, a new era and the world is changing fast.”

Watch the special Christmas preview here.

Call the Midwife, BBC1, Christmas Day, 7.50pm