Rosemary Rawlins had always known she was swapped at birth - but is not prepared for the shocking details.
Desperate to find any biological family she may have, or the girl she was switched with, the 77-year-old got in touch with the Long Lost Family team.
Her quest for the truth is the stuff of fairy tales, but also the stuff of nightmares, with huge repercussions for everyone involved.
Rosemary doesn't remember exactly when she knew she was different, but recalls a "peculiar" moment during a family holiday when she was a child.
"The first time I can remember Mum saying something, which when I look back on it was very peculiar, we were on holiday in Weymouth," she explains on tonight's episode.
"We were actually in the theatre and there was a lot of noise and I just remember Mum saying, 'Oh that family down there, that could be your family'.
"I sort of looked and thought, 'Yes, there's a girl there with a mum and dad', and that was it. I didn't think about it anymore, the show began, and it never worried me. I just thought of it as a fantasy story."
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Rosemary, who now lives in Sussex, always felt incredibly loved growing up and was the eldest of three siblings.
However, she was always aware of differences as she was tall, athletic and tanned easily, compared to her slim, fairer siblings.
Any thoughts of an alternate family were put to one side when her mum and dad died early, so she took on the role of being head of the family.
It was only decades later while making a photo album for her sister that the questions bubbled up again - and DNA test results revealed what she had always suspected.
Rosemary, who always felt like "a cuckoo in the nest", discovered that she wasn't genetically related to her family.
She says: "When I was young it didn't matter to me at all. It's much better to get on with life. Until you know that something can be done — and then it changes everything."
While speaking to her cousin Nicky, Rosemary discovers the full details of the unfortunate mix-up between Rosemary and another baby.
Nicky says she was always told that Rosemary was born in a nursing home in Weymouth and was "accidentally switched" with another baby.
"Granny told me the night you were born there was an air raid and the babies were all taken down into the basement because they couldn't be fitted with gas masks. While the mothers stayed on the ward.
"Granny said when she came to visit the next day she said, 'This isn't the same baby'."
Initially, the family were told that their baby looked different because of jaundice but they realised something was wrong.
Nicky continues: "Apparently he family doctor was called in and consulted and he said to them, 'The logical conclusion of this is that you might have to give the baby up'. But they were not prepared to give you up.
"For Mum and Granny it was a regular topic of conversation. They would say not to mention it even in front of you Mum as she would get upset about it."
Rosemary says she has a wonderful family and doesn't want to lose any of them, but has the right to know who her biological parents were.
Researchers from the Long Lost Family team have never come across such a complex case - and it's so unprecedented there are no laws surrounding how to reveal the truth to someone.
There are additional implications as Rosemary's family could find their biological sister, while the other woman involved may not even know she was switched at birth.
They discover a woman called Jackie, who was born at the same nursing home as Rosemary just three days apart.
Jackie had no idea she was not biologically related to her family - and DNA test show that her brother Richard is Rosemary's full sibling - proving there was a switch.
Richard, who lives in the house that his parents used to in Weymouth, admits it has been a shock for the family and they didn't even have an incling about the mix-up.
Richard says he and Jackie are "like chalk and cheese" but have an incredibly close bond - and he is also very keen to meet biological sister Rosemary.
While an emotional Jackie confesses: "It's been a lot to take in. There are days when I think, 'Yeah, these are my family, but are they my family?'
"That's what goes through my mind when they say, "You're our Jacky and we love you and you'll always be our Jacky'."
Rosemary is thrilled to discover that her biological brother wants to meet up with her - and sees a photo of her genetic parents for the first time.
"She knew that I wasn't her baby and somehow she had to accept it and that I find incredibly sad," she says about her mother, but the truth brings her comfort.
"Seeing pictures of my genetic mother and father just made me feel very whole. That these two lovely, charming people sitting together were my family."
Rosemary, who feels lucky to now have two families, meets Richard for the first time at the theatre where her biological mother worked for many years.
"It was just fantastic meeting my brother at my great, old age and it was utterly delightful," says Rosemary.
Richard says Rosemary and his mum are "kindred spirits" and they may have crossed paths at the theatre.
"I did feel very much for Richard's sister who is obviously finding this a very tough time and we'll have to be very slow and very gentle and caring," says Rosemary.
"It has bene just fantastic. I have got a new brother and I thank him from the bottom of my heart that he was able to do this for me."
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