It will be Call The Midwife’s most controversial storyline yet – and one of its most heartbreaking.
Next Sunday more than 9million viewers will see how a romance between a disabled man and woman, and the baby it produces, causes disgust in 1950s London.
The couple, who live in an institution, are immediately separated from each other. And the love between Jacob Milligan, who has cerebral palsy, and pregnant Sally Harper, who has Down's Syndrome, is branded unnatural.
The discomforting plot, inspired by writer Heidi Thomas’ personal experiences, will shatter the usual easy, heartwarming pace of the BBC show.
And today the disabled actor and actress at the centre of it reveal how, even in our supposedly more enlightened times, there is still discrimination to overcome.
But in an uplifting interview, Sarah Gordy and Colin Young say their roles in the hit series have left them grateful they were not born in post-war Britain.
“It made me feel such anger when I read the script,” says Sarah, who has Down' Syndrome. “Back then people did not want to admit they had a disabled person in the family. They would say it was ‘bad blood’.
“I know how lucky I am because I’m not institutionalised like they were then. I live with my family in Sussex. I do my acting and I work as a Mencap ambassador.
“I get to have my own life and I don’t see myself as different from anyone else. In those days they were treated as though they were incapable of making their own decisions – even when it came to relationships.”
Sally falls in love with Jacob, played by Colin Young, after both are sent to an institution by their parents. Jacob is ridiculed by Sally’s mother and father for getting her pregnant.
And Scottish actor Colin, 27, says he knows just what it feels like to be judged by strangers.
Colin, who grew up with cerebral palsy in Edinburgh, recalls: “When I was six I was walking down the street with Dad and someone shouted, ‘why would you bring someone like that out with you?’
“My dad told him: ‘He’s actually my son’. I remember things like that because they’re hurtful.
“But I went to a mainstream primary school and spent a few years at a secondary and I wasn’t bullied there.
“It was hard because I had to watch from the sidelines as people played sport. I wasn’t included in everything, but I’ve always strived for my independence.
“It’s what I value most in life. As a kid, I never realised how hard it would actually be to achieve. Just getting the support and practical things I need to live independently takes a lot of patience. But it’s what I always strived for.”
Old-fashioned attitudes towards sexual relationships between disabled people have moved on, but Colin says it’s still not an easy subject matter.
That’s why both he and Sarah, 25, are glad Call The Midwife’s writer Heidi Thomas was keen to tackle such a difficult love story after experiencing disability first-hand when her late brother was born with Down’s Syndrome in 1970.
And they have called on other TV shows to tackle similarly sensitive issues – to help end discrimination for good.
Colin says: “Heidi told me she wanted to write this episode for a long time.
“She wanted to express the difficulty of disabled people wanting to find love. I would say attitudes are still similar towards sex now.
“I’d like to find a partner and have a family of my own one day. My relationships have mostly been with other disabled people, because I've found there’s often a kind of mutual understanding of life but it all depends on the person.”
Sarah says that unlike Sally she has no desire to fall pregnant.
She denies it has anything to do with her Down’s Syndrome – a genetic disorder affecting around 1 in 1,000 babies in the UK.
She says filming her pregnancy scenes was made easy by “warm and friendly” Miranda Hart (Chummy Noakes) and Bryony Hannah (Cynthia Miller).
Sarah adds: “Miranda was the biggest hoot. She wandered in from make-up one day wearing a thin midwife’s uniform and with one leg of her Long Johns down one ankle and the other hitched above her knee.” But playing the part didn’t change Sarah’s mind about having a baby of her own.
She says: “I’ve never wanted one. I have friends with disabilities who have families and it is so tough, but it’s not even about that. I would just prefer to focus on my career.”
Jacob, who studied politics at Loughborough University and now lives in a flat in Glasgow, wants more TV dramas to involve disabled actors.
“I am just a part of society and that should be represented on our screens,” he says. “I am so glad Call The Midwife isn’t afraid to tackle sensitive issues.”