IF she's honest, the film director Haifaa Al Mansour admits, she wasn't sure why anyone would think that she was the best person to make a film about the life of Mary Shelley.

After all, the teenage author of that Gothic masterpiece Frankenstein grew up some 4000 miles and some 200 years in distance from Al Mansour's own childhood in Saudi Arabia. And as for the idea of shooting a costume drama in a foreign culture, well, Al Mansour admits, "I was intimidated by that."

But then she read Emma Jensen's script and she realised that her world wasn't so very far apart from that of Frankenstein's creator. Both women grew up in societies where to be a woman was to be a second-class citizen.

When Shelley, born in 1797, published Frankenstein in 1818 she did so anonymously. The only name on the book's cover was that of Percy Bysshe Shelley, the Romantic poet, Mary's lover and, Frankenstein's readers assumed, the true author of the book.

Two centuries later, when Al Mansour was making her debut film Wadjda in Saudi Arabia, she had to hide away in a van when the film was being shot only communicating by walkie talkie because men and women are not supposed to mix in the workplace

Patriarchal conservatism crosses centuries and continents.

It's a Monday morning in Los Angeles. Al Mansour has just packed her kids off to summer school and now she is talking to me about Mary Shelley and the film she has made about her.

"I felt so sad that I didn't know much about her," Al Mansour says. "She didn't get the recognition she deserved only because of her gender and her age. I really sympathised with the character. I could feel her pain and hurt so I decided to get involved."

The resulting film, Mary Shelley, is costume drama as feminist cinema. Starring Elle Fanning as the author, Al Mansour's film is a handsome period piece full of desire and death and (sometimes clunky) name-dropping. ("That's Shelley," Maisie Williams tells Fanning near the start of the film. "He's a radical poet who thinks poetry should reform society so is often in trouble.")

That said, it's a film that tries to look beyond reputations. Al Mansour didn't want to make the poets Shelley (played by Douglas Booth) and Lord Byron (Tom Sturridge) larger-than-life characters. She wanted to see them as people.

"Especially Byron who was very sexist and his ideas about women were sometimes ridiculous and silly," she says. "I wanted to judge them not as poets but as a group of young people coming together and all the dynamics between them."

That dynamic played out on set too. Shooting with all her young actors in a villa in France standing in for the Swiss villa where Mary first came up with the idea for Frankenstein in the summer of 1816, al-Mansour recalls the producers coming to her and saying "'Haifaa, control this set. It's a madhouse in here.'"

Really? "The actors were having fun," she says.

Mary Shelley's life story is remarkable and tragic. Her mother, the writer and philosopher Mary Wollstonecraft, died shortly after her daughter's birth. Mary herself ran away from home at the age of 16 and lived with Shelley in a state of constant debt and scandal. She was still a teenager when her first child died. Two more infant children would die in the following years before the poet's death by drowning in 1822.

That's a lot for a young actor like Elle Fanning to take on board. But her performance is one of the quiet strengths of the film.

"I think Elle was amazing," the director agrees. "Her performance was very subtle and very elegant. This all could have been melodramatic. Her baby dies and she's always arguing with Percy and her relatives. A little bit of overacting could push it the other way. But Elle's very subtle. She's very young but has maturity, so she embodied everything we wanted to say about Mary Shelley."

The film takes its tone from Fanning's performance. It eschews the lurid surrealist melodrama of something like Gothic, Ken Russell's 1986 film on the birth of Frankenstein, for a more restrained approach (no eyes in nipples here).

But what starts as a conventional romantic drama gets tougher and tougher as it goes along. By the final act, as Mary confronts Shelley over his failings it feels quite angry, I tell Al Mansour.

She's not certain about that. "It's not an angry film, no. But there is a sense of frustration in the film, especially when she goes to try to publish her book and she is rejected. Why can't this young girl publish her book?

"Percy and Byron come with this stamp of approval from the public, right? That's why the publisher wants Percy's name. The intellectual property was dictated by the publishers and who they think they can sell to the public."

She returns to my original question. "I did a lot of writing [on the script], especially in the third act, so, if there's any anger, it's coming from me."

Al Mansour, now in her early forties, grew up in a small town in Saudi Arabia, one of 12 children, in a country where cinema was banned. But she spent her childhood watching videos. Like Quentin Tarantino, you could call her a director of the VHS generation.

"We got a lot of films from the video store and I fell in love with them," she says. "I felt I belonged to a bigger world than my small town. I wanted to make one."

She started shooting short films with her brothers and sisters. "It was almost like a family video. My brother was holding the camera and my sisters the light. We were always arguing. My family are like Italians.

"So, it wasn't something very professionally done but it got accepted and gave me the sense that I exist, and I matter."

Well, yes, but the idea of a woman making a movie in Saudi Arabia wasn't popular, was it? "I started making films in 2005 and people were making fun of me and saying that I'll never succeed. That's why I sympathise with Mary Shelley's journey when she gets creatively dismissed and how it hurts," she says.

"But now Saudi Arabia has changed a lot. My first short went viral and I started getting death threats and stuff, but now it's totally different."

It's tempting to ask her to parse that "and stuff" comment, but she's still talking about how much has changed in the land of her birth. "They are opening cinemas and women are able to drive. It has changed a lot since I started making films."

She's going back in September to shoot another Saudi movie. Her first, Wadjda, in 2012 became the first Saudi film submitted to the Oscars. Mary Shelley marks her move into English-language films and she's keen to make films in Hollywood too where she's now based.

"It's an exciting time now. They're pushing for diversity and they're pushing for women, so I feel there's a place for me, whereas if I came five years ago I think it would have been more difficult to find jobs. I want to be part of the change happening here."

She's said in the past she'd even love to shoot a big-budget superhero movie, I remind her. "Working at it," Al Mansour says, smiling.

But for now, she has Mary Shelly to promote. Given the restrictions she has had to work under in Saudi Arabia, what was it like, I wonder, having freedom to shoot. Her film even has love scenes.

That was a challenge, she admits. "I'm very shy. Intimate scenes don't come naturally to me. Unlike argument scenes."

But the freedom is intoxicating, she adds. "I think western film-makers take it for granted. I was really happy to be engaging with my art and not to be worried about what we say or are we shooting in a conservative neighbourhood."

Telling stories, she says, can make people more tolerant. Cinema is a political act then, Haifaa?" "It is a way you can influence without being militant. I think it's very important in conservative places where people shy away from activism and they don't like loud voices, right? Art is an amazing medium to reach people that way."

No doubt Mary Shelley would agree.

Mary Shelley opens in cinemas today.