Call the Midwife star Laura Main has teased a "complex" storyline in the Turner household in the upcoming Christmas special.
The festive episode begins two weeks before the big day and sees Shelagh and Patrick Turner receive a package from their foster daughter Mae's biological mother in Hong Kong.
When asked about the emotional storyline and Mae's place within the Turner family, Laura told HELLO! and other journalists at a recent press event: "It's never been resolved, and [Mae's] so part of the family that I think a lot of the audience think that we have adopted her already. We would in a heartbeat but haven't been able to.
"They carry on with family life and make it as normal as possible for her. They're her brothers and sisters and we're her mum and dad. It's complex."
Stephen McGann, who plays Patrick Turner, added: "Let's just say, it's a reminder that things haven't been resolved."
Laura also added that the Christmas special marks a change for the actors who play their on-screen children. The Scottish star explained that April Rae Hoang (May), Alice Brown (Angela) and Ned Shaw (Teddy) will have more lines in the script.
"It's nice to see that family environment," said Laura. "Now the kids get lines. That's the big thing this year for our family unit. They've got quite a bit of dialogue, proper acting stuff, it's great."
"You're getting Christmas through the lens of a family."
Elsewhere in the festive episode, Nancy Corrigan decides to stay at Nonnatus House after receiving a job offer for a nursing role in the countryside in the series 12 finale, which left her future in Poplar uncertain.
Actress Megan Cusack told HELLO! and other members of the press: "Me and my daughter move into Nonnatus House. We have our own little bedsit, which is very sweet."
The upcoming special will no doubt be a tear-jerker, as always, and will see Sister Monica Joan in low spirits as she declares the 1968 Christmas to be her last.
The synopsis teases: "Sister Monica Joan (Judy Parfitt) is convinced this could be her final Yuletide and decides she'll not live to see man walk on the moon. The Nonnatus Family, including Trixie's (Helen George) brother Geoffrey (Christopher Harper), decide something must be done to try and lighten her state of mind."
Jenny Agutter, who plays Sister Julienne, revealed that her character will be on hand to lift Sister Joan's mood. "It's up to Sister Julienne to help her see that there is no reason to believe these dark thoughts and that there is a great deal of vitality in her," she told Digital Spy.
Jenny also revealed that the episode builds to "an extraordinary end" as the community joins together.
"One of the things that fascinated me is because we've been away places such as South Africa and to the Outer Hebrides, and we've done a couple of them that have been very contained," she told HELLO! and other journalists. "It's always interesting to me that it can actually have so much within it when it's contained in that way. And how important the community becomes and how all of those stories weave together.
"It builds to an extraordinary end," she added.