An unsettling atmosphere washes in and out with the tides. Something terrible has happened on the island and the wild beauty of the coast has a troubling undertow.

Polly Crosby's latest novel, The Unravelling, is set on an island drenched in secrets. The solid physical geography of sand dunes and shingle spits, concrete bunkers and blast shelters, is inspired by Suffolk’s Orford Ness. But this semi-fictional island is off the coast of Great Yarmouth and Lowestoft.

Polly calls her island Dohhalund from the Dutch name for Doggerland which once joined East Anglia to Holland. “I like to change things, start with something real and then twist it to make it my own,” she said.

Polly grew up in Walberswick on the Suffolk coast and now lives near Norwich. Part Norfolk, part Suffolk, her lyrical writing is rooted in the land and seascapes, history and stories she has loved all her life.

Polly has her own remarkable story. She was the first baby in the world to be diagnosed with cystic fibrosis with the heel-prick test.

Born in Ipswich 1980, she was three-months old when her parents were told their third daughter would not live beyond childhood.

Polly’s mother, a nurse, already suspected something was wrong with her baby, when the results of the heel-prick test at Ipswich hospital revealed she had cystic fibrosis.

Polly was not expected to celebrate her 10th birthday – but despite regular chest infections and hospital stays she was kept alive with a regime of physiotherapy, nebulisers and intravenous antibiotics. Remarkable new drug treatments meant she survived to marry, have a child and become an award-winning writer.

“Advances in medications have been just incredible,” said Polly. “There have been some amazing tablets. The latest ones I started a year ago and they’ve transformed my life. I still have CF but it’s enabled me to focus on everyday things, the little things that normally I would have to plan and worry about. I can write for hours every day. I don’t need iv antibiotics. All these things mean my life is wonderful.”

In The Unravelling, Tartlelin is hired to catch butterflies for an elderly woman, but is soon searching for clues about what happened to her employer, Marianne, and the island where she lives, many decades ago.

The island is inspired by the wild coastal spit near where Polly grew up. “Orford Ness was used in the 20th century by the military and I loved this idea of mankind making the land there its own and the scars still being there after they have left. I liked the idea of secrets buried beneath the surface. We still don’t know exactly what went on there in the 20th century,” said Polly.

Her island also features herring fishing and the herring girls, shifted across the sea from Yarmouth. “I went to Time and Tide Museum many times during my research,” said Polly.

A story of a life devoted to collecting and studying butterflies flows alongside suggestions of strange mutations and mermaids, submerged mysteries and memories. There are moths and silk-making too and as Tartelin tugs at loose threads, in Marianne’s house, in the people she meets, beneath the shifting sand and the surrounding sea, she pieces together a story of tragedy, romance, loss, betrayal and healing.

Polly’s writing is fluid and poetic, the dual time-line plot ebbs and flows between Marianne’s childhood and old age, throwing up tide-lines of Tartelin’s past. The story is of butterflies and silkworms, of flying free and being pinned, of characters and landscapes concealing layer upon layer of tightly wound secrets which eventually unspool in a series of dramatic metamorphosis, both actual and metaphorical.

Repeating patterns of time and plot are as symmetrical as a butterfly’s wings, with an emotional and stunningly-written denouement which reveals life can be as fleeting.

Despite her illness Polly had a normal, happy childhood “My parents treated me exactly like any other child,” she said. “I wasn’t wrapped up in cotton wool and I was allowed to go out in the Suffolk countryside and get muddy and forget to wear my coat, like any other child.”

But knowing that her time might be limited helped her focus on what she really wanted to do.

“It has always made me want to push to do things as quickly as possible and plan ahead in the hope that I will the things done while I’m well enough. It made me want to get a book published because I knew the chances were going to be very slim but if I was going to anything with whatever years I had, that was my top priority that’s why I kept trying and kept trying.”

When she was too ill to stay at university she returned home and wrote her first (unpublished) novel at just 19. She applied for a place on the University of East Anglia’s creative writing course but was turned down, and told to reapply when she had more life experience.

Almost two decades later, by now married and a mother, she won a scholarship place on the course. Her first published novel, The Illustrated Child, won awards last year was called ‘a classic in the making’ and ‘an extraordinary debut.’

The haunting coming of age story is set in 1980s Suffolk where Romilly lives with her eccentric artist father. As he succumbs to early-onset dementia Romilly realises that his picture books contain a hidden treasure hunt, just for her. But this is no fairytale and she unlocks the disturbing secrets of their past in another watery landscape of rivers, reeds, water meadows and marshes. “There’s something about that sort of landscape which really is close to my heart,” said Polly.

The Unravelling features art too and Polly said: “I always wanted to be a writer or an artist. I tend to write about artists. I think it’s stitched into me. My first book was about an artist who created a beautiful picture book. In The Unravelling it is not such a big theme but it is still important part of the novel.”

She is also an artist and turns her miniature paintings into jewellery. “I’m making little hand-painted necklaces, of butterflies and bees and animals and I set them in resin in silver pendant. I sell them on Etsy,” she said.

Polly met her husband as she moved house in Norwich. “I locked myself out by mistake and the only person I knew in the area was this nice antiques dealer I had met the day before because I wanted to buy some furniture. I thought I’d go and say hello to him and tell him my predicament,” she said. “He’d just left his workshop but saw me and turned his van round, pretending he’d forgotten something and I told him what had happened. He said, ‘I’m just about to go to Southwold for chips and a walk on the beach do you want to come?’ And I got into this unknown man’s van and the rest is history!”

“I do remember at one point we were talking so much that he went the wrong way and I suddenly though oh my goodness where’s he taking me?”

Polly and her husband, who is still an antiques dealer and furniture restorer, now have a 14-year-old son. Shielding for much of the pandemic, as someone with a serious lung condition, Polly kept writing. “The pandemic was a tricky time but I was so much more fortunate than many because I could carry on writing. I didn’t step outside my front door for months.”

She finished her third novel, a historical dual-timeline mystery on the Suffolk coast featuring a young woman who breaks into a derelict mansion and discovers a portrait inside that looks just like her - but was painted 40 years before she was born. Her next book is set on the Norfolk Broads with a dual 1912 and 1950 timeline.

“I feel like I’m half Norfolk, half Suffolk,” she said.

Myth and reality, land and sea, past and present merge in her writing and her novels are alive with East Anglian folk stories too – a woman weeping for her dead baby at the ‘shrieking pits’ of Aylmerton and Northrepps, a black beast prowling through pages. “When I write I go back to my childhood in Suffolk because it was such a beautiful place to grow up,” said Polly, who loved her primary school in Henham and then middle school at Halesworth, where she particularly remembers inspirational English teacher Miss Pirrie. Her parents have also always been hugely supportive. Her mother was a nurse; her father a customs officer. “They are my greatest champions. My mum is constantly talking to people about my books!” said Polly.

When she received an advance as part of a publishing deal she created bursaries to pay for writers who are disabled or living with a long-term illness, to enter their novels in The Bridport Prize - for which her first novel was a runner-up.

Her second, The Unravelling, is washed through by her lifelong fascination with the sea and its power to both connect and isolate. She learned to swim at Walberswick and said: “I’m very much a sea swimmer and I love just looking at the sea too. I love how it changes minute by minute. The sea is in my blood and always will be and now that I’m in Norfolk I’m always at Norfolk beaches,” said Polly. “I love Sea Palling and Winterton, those huge stretches of sand and vast skies. This summer I swam at Horsey and young seals were really inquisitive. I think they thought I was a seal!

“I feel very lucky in many ways. Having cystic fibrosis has made me seek out the wonderful in the mundane and there is so much wonderful in my life.”

The Unravelling, by Polly Crosby, was published this week by Harper Collins.