“Oh my god. Like, thousands. Definitely in the hundreds. Probably more.”

Ruth Wilson has told this story many, many times before. If you’ve spent time in a pub, a rehearsal room, on location or at dinner with the Golden Globe and Olivier Award-winning star, she has probably mentioned the incredible story of her grandparents. Now, she’s telling the world, as producer and star of Mrs Wilson on BBC One, in which she plays her own grandmother, Alison Wilson.

It is weird, I have never got bored of it. People always go, ‘What? Wow! Really?’ That is what reassured me in telling this as a drama. A good story needs to be told.

Since bursting on to the scene with her Bafta-nominated lead role in Jane Eyre back in 2006, Wilson has established herself as a major talent. She headed a strong cast featuring David Oyelowo, Benedict Cumberbatch, Naomie Harris and Ashley Walters in the BBC adaptation of Small Island, stole every scene in Luther as psychopathic mass killer Alice Morgan, and won awards for big US hit The Affair alongside Dominic West.

But all the while, Wilson was sitting on a true-life family story far stranger than any of that fiction.

In 1997, she found out that her father’s father, Alexander Wilson, had a secret second family. So not only was he involved in espionage, fluent in Arabic, and the author of numerous spy novels, but he had also been married to someone else for the entire duration of his 20 years with Wilson’s grandmother. And, after discovering his deceit following his death in 1963, Alison Wilson kept the secret from her teenage children until they had teenagers of their own. One of them was Ruth.

Mrs Wilson

At a screening the night before our interview, Wilson looked unusually anxious. These are usually celebratory events, a chance for cast and crew to share the finished work in public. But at Mrs Wilson, the audience included Wilson’s father, uncle and members of her extended family.

“I was so nervous about their reaction, because it was the first time they had seen it,” she says. “I knew watching their family story must be an odd experience, whether it is moving or just discombobulating. I was very sensitive to how emotional they might be feeling in this big public arena. I didn’t care what anybody else thought!”

A long dinner with very close friends and family, a few toasts and a major debrief later, Wilson is happy to report that her drama passed its most important test.

“As my career grew, people were more willing to listen to this story and the BBC were more willing to take a risk on me. The stars all aligned at the right time. It is something I will be very proud of forever. It does feel like a gift to my family.”

And the Wilson family extends way further than even her grandmother realised during her lifetime. While the dramatisation is largely based on her memoirs, there were more revelations to come. Further investigation revealed that she had, in fact, been the third woman to marry Alexander – and that he had married again before his death. Four weddings, no divorces, seven children. These days they all meet up.

A real, true domestic drama that is deeply personal,

Crucially, the drama is not your classic James Bond fare about the extraordinary life of a spy. Instead, it focuses on the effects of his secretive life on those closest to him. It’s stylish, beautifully acted and, at times, jaw-dropping. The cast includes Game of Thrones star Iain Glen as Alexander and Keeley Hawes as the mysterious Dorothy, while Fiona Shaw follows Killing Eve with another star turn as an intelligence chief. In the middle of it all is Ruth Wilson, stepping into her grandmother’s shoes.

“I think it is unique. A real, true domestic drama that is deeply personal, but also full of intrigue. It feels like a domestic thriller,” she says.

“It has scale to it but really it is about this woman, her journey and what, on a very intimate level, she is going through. Her life was really ordinary. But she married this extraordinary individual and through that her life became extraordinary. It makes you feel that maybe everyone’s life, in close up, is extraordinary.”

Mrs Wilson is at heart a story of stoicism and resilience and fortitude. On screen, we see Alison working as a secretary during the war and meeting the dashing older intelligence officer. They were married soon after his supposed divorce and a bombing raid on London that sent her running into his arms had cleared the path to romance. Two children followed.

We also see her, 20 years later, realising her life and love were, at least in part, built on lies and uneasily negotiating funeral arrangements with the secret half-brother of her sons.

“I have so much affection and love and understanding for my grandmother and members of my family, and the women involved. All the wives who dealt in their different ways with this husband and the fallout from his actions,” says Wilson.

“How strong they all were, how protective they all were of their own children. They all had some sort of code of conduct where they always put their kids above themselves, above their anger and fury – the idea that no matter how betrayed and upset and devastated they were individually, they did everything to protect their children from the truth.

“The question in the piece is whether that is a good thing or not. Is it good to protect your kids from this information or to tell them everything?”

To this day, Wilson has been unable to find out all the details of her grandfather’s life.

“After 70 years, MI5 still won’t release his records as to what he got up to. They’re ‘case sensitive’, whatever that means,” she says.

Yet she is adamant that her grandfather was no straightforward cad. He seems to have been universally loved, quiet, sensitive, well-read, charming. “I wasn’t convinced he was marrying these women for intelligence reasons. I felt he did fall in love with these women. It didn’t feel like a cynical act.”

For Wilson, this family drama marks a new stage in her career. A new beginning after a few intense years and the discovery of a new way of working.

“Acting is brilliant, but it has felt empowering to also be on the other side of the camera. It is so much more full as a process.”

DID YOU KNOW…

Since 1991 The Big Issue has sold more than 200,000,000 copies – helping the most vulnerable in society earn more than £115 million.

The next step? “I plan to have more fun,” she grins. “I’m filming His Dark Materials at the moment, flying to Cardiff, doing a bit of acting with a monkey, flying back again. It is all fantasy and madness, great outfits. I’ve never done fantasy before. A mix of Star Trek and Doctor Who – so that is new.

“Then I am going to do King Lear – playing both Cordelia and The Fool on Broadway. So I get to be a wally on stage for a bit. I get to prat around. I have never done Shakespeare professionally, so that is another new thing. Tick it off, don’t have to do that again!”

Mrs Wilson starts on November 27 on BBC One