Londoner Irena Brignull has a long track record as an in-demand screenwriter, including work on the Oscar-winning Shakespeare in Love, as a script executive at the BBC, and, most recently, penning the screenplay for the Netflix adaptation of The Little Prince. Writing a novel was on her wish list, but with screen projects lined up like planes waiting to land at Heathrow – and three kids – she had neither time nor mental space to commit to it.

Then her youngest son came down with an illness that required a long hospitalization, and Brignull put her career on pause while he recuperated. “It was after that, when a friend invited us to join them on holiday and I was watching [her son] run and play outside, that I relaxed and felt a freedom to do something else,” said Brignull.

The result is The Hawkweed Prophecy (Weinstein Books, Sept.), a novel about two teenagers – one a witch, one not – that blends fantasy and feminist thought. “The idea of witches has always been interesting,” she said, “but when I started to write I found I was more interested in the notion of powerful women, women who wanted independence and were punished for it, than I was in the magic.”

PW’s review called the debut novel “a fantasy with the air of a classic,” and foreign rights sold briskly after Orchard in the U.K. bought the book at auction in January 2015, said Brignull’s primary agent, Catherine Clarke of Felicity Bryan Literary. Clarke reports sales to date in France, Germany, the Netherlands, Italy, Portugal, and North America – where Brignull is represented by Catherine Drayton of Inkwell Management. Weinstein Books’ Amanda Murray acquired the book here, and former Little, Brown Books for Young Readers executive editorial director Cindy Eagan edited it.

“There was a day when the phone kept ringing and the news was, ‘Oh, I sold it to Germany,’ and then, ‘It sold in France,’ and then, ‘Holland, too,’ ” Brignull said. “All that when I still couldn’t quite believe I had sold it at all.”

The two girls at the heart of The Hawkweed Prophecy, Poppy and Ember, have both been victimized by a jealous witch worried that baby Poppy might be the witch prophesized to one day lead the coven; she concocts a spell to switch Poppy with a baby born to commoners. As a result, Poppy grows up not knowing her heritage but constantly, inadvertently, causing chaos as her powerful magic leaks out in destructive ways. When her father finally moves her far away from the schools from which she’s been expelled, Poppy has a chance encounter with Ember, whose childhood among witches has been a disaster since she shows “no predisposition for magic whatsoever.” Both girls feel like outcasts without knowing why.

For the author, the transition from screenwriter to novelist was not without challenges. “I would work really hard all day and have one page written,” Brignull said. “With a screenplay, so much of it is dialogue, and so much of the work is done in figuring out the structure. I had to get used to a much slower pace and I had to learn patience.”

Though she didn’t start out writing novels, Brignull has a degree from Oxford University in English literature and a lot of her work for the screen has had a decidedly literary bent. In addition to adapting Antoine Saint-Exupéry’s classic novel, The Little Prince, she also served as script editor on I Capture the Castle, the 2003 film adapted from Dodie Smith’s novel, and wrote the screenplays for the adaptations of David Almond’s Skellig (2009) and The Boxtrolls (2014), based on the novel Here Be Monsters! by Alan Snow.

Though a pedigree like that ought to inspire confidence, she showed her first draft to her oldest daughter – then only 11 – with trepidation. “When I first started writing it, I thought it might be just for my kids,” Brignull said. Her daughter set her straight. “I knew to take her response with a pinch of a salt because she’s my daughter, but she was an amazing pre-reader, and she was very encouraging that it ought to be a series.”

And although Brignull says she suspects the novel sold quickly in the U.S. because she was already known to the Weinsteins, the producers of Shakespeare in Love, her agent, Clarke, thinks Brignull’s storytelling speaks for itself. “Our German co-agent, Sabine Pfannenstiel [doesn’t] think Fischer [the book’s German publisher] was swayed by her screenwriting credentials. They simply fell really hard for the text and were quick to pre-empt.”

Brignull has already written a sequel, The Hawkweed Legacy, which Weinstein will publish in fall 2017. All the attention that her debut novel received even tempted her 12-year-old son to read it. “He was not willing initially but he did read it and he gave me the biggest compliment,” Brignull recalled. “He admitted that, by the end, he almost had a tear in his eye.”

The Hawkweed Prophecy by Irena Brignull. Weinstein Books, $18 Sept. ISBN 978-1-60286-300-8