IT IS A TRUTH universally acknowledged that a novelist in possession of a plot must be in want of an opening sentence -- and this is the only place to begin an interview with Helen FitzGerald, whose novel, Viral, opens with six words that will not bear repetition in a family newspaper.

Suffice to say that that first line describes a sex act performed by a teenage girl on a number of men in a Magaluf night club.

Forty-nine-year-old FitzGerald -- the prolific, bestselling author of 2007‘s Dead Lovely and nine other adult and young adult thrillers, including My Last Confession (2009), The Donor (2011) and The Cry (2013) -- enjoys tackling taboos. Dead Lovely, for instance, has an attention-grabbing opening and The Cry starts with a dead baby. FitzGerald’s harrowing subject matter ranges from infanticide to death, ageing and sinister happenings among dementia sufferers in a care home in The Exit (2015).

In Vral, a film of the aforementioned act goes viral. The book deals with the subsequent shaming, abuse and intimidation of the young woman and her family. As well as exploring the overwhelmingly misogynistic nature of the minefield that is social media and the misery left in the wake of online shaming, Viral is a perceptive look at mixed-race adoption, the relationships between a formidable mother and her daughters, sibling rivalry and bullying. There was a “hotly” contested auction for TV rights, which went to Kudos, producers of Broadchurch and The Tunnel. First, though, there will be a BBC-tv version of The Cry, of which the script is currently being finalised.

Nonetheless, Glasgow-based FitzGerald’s publishers are promoting Viral as her “breakout” book, despite the fact that several of her previous novels have been longlisted for many awards, from the Theakston’s Old Peculier[correct] Crime Novel of the Year to The Guardian’s Not the Booker Prize.

Faber & Faber obviously have a great deal of faith in Viral -- tipped as “the next Gone Girl” in the so-called “domestic noir” genre -- but Australian-born FitzGerald confides, when we meet in a Glasgow hotel, there have been endless, indeed ongoing, debates about that vexed first line.

Was there pressure to change it? “There’s still pressure,” she confesses. “They wanted me to take it out for the Trade Paperback edition because supermarkets won’t like that sentence. My gut feeling is, yes it's shocking but it’s quite a poetic line; it says what you need to know. They are still saying that when the cheaper paperback edition comes out, which the supermarkets may or may not take, it has to come out. I will see what readers make of it. It is not my publishers, who are quite open-minded, it’s the supermarkets who won’t want it on their shelves. They seem to have no problem with books about women being raped or tortured, or a child being killed, but you can’t have that line. I’m a feminist and my books are feminist. That opening line is feminist because it’s about a woman taking ownership. It’s been a very contentious issue but I’ve stuck my neck out. I’m keeping that opener.”

She was in Harrogate, in 2013, at the crime writers’ festival, “which is quite a sociable, drinking affair,” when she thought of that sentence.

“I remember running up to my room -- quite drunk! -- and writing it down. If I get the first sentence or one paragraph, I've got the book. You have the character, the voice and what they’re talking about. I went back downstairs and was among the last to stay up -- as usual. When I woke the following morning, I saw this piece of paper by my bed and thought, ‘My God!’ But I went home and just wrote the book very quickly.”

FitzGerald works as a criminal justice social worker -- a job she loves but which she gave up several years ago to become a full-time writer. She is married to BAFTA award-winning, Scots-Italian screenwriter Sergio Casci, with whom she has a teenage daughter Anna, and a son Joe. Pleasurable as it was to work at home, she missed her day job so much, she has returned part-time.

Did her professional background influence her decision to write about mixed race adoption? (The central figure of Viral is 17-year-old Korean Su-Jin, adopted by the Scottish Oliphant-Brotheridge family -- that formidable mother is a high-flying, respected Sheriff.)

“As a social worker, obviously I’m interested in the issue,” FitzGerald replies. “But for a long, long time, before I even became a social worker, I had wanted to write about it. When I was at uni (she read English and History at Melbourne University), I worked in a sandwich bar. A heavily pregnant woman came in one day with this beautiful, dark-skinned baby she had adopted. She was besotted with the child and we all ‘ooed’ and ‘aahed’ over it.

“The next time I saw her she was on the TV news. Social workers had taken the baby away because she was pregnant. They said she was going to have a white baby and had taken on too huge a responsibility with a mixed race child. I don’t know what happened to her or the baby, but then my brother adopted a little girl from Ethiopia, who is nine-years-old now. So I was always interested in the subject. I like to write about family dynamics that are a bit different -- which may be something to do with being the 12th of 13 children. But that and the first line was all I needed to fly.”

When FitzGerald started Viral, her daughter was flying to Magaluf on holiday with six friends on leaving school. "There had been story after story after story about girls who have been filmed over there doing all sorts of things. The debauchery. Women being treated like meat. At first, I was terrified for Anna. But I really trust her -- and I've dedicated the book to her. She helped a lot with the atmosphere of the place, the routine of the day. Even just seeing her pack a ridiculous amount of bikinis!

“I watched a lot of reality TV, such as Geordie Shore and Magaluf Weekender. Horrendous programmes! Everybody is sleeping with everybody -- I feel very old-fashioned when I watch this stuff. Also, I read Jon Ronson’s excellent book, So You’ve Been Publicly Shamed [which explores how some people’s lives have been ruined by reactions to ill-judged social-media confessions or clumsy tweets] and I also read up about the Dark Net. Scary stuff!”

FitzGerald first used social networks when she wrote her first book. “I was told I had to write a blog, get an author Facebook page, get on Twitter and -- until this book, which has changed me -- I was very open, blogging about my personal life. I’m always open with people; I don’t like secrets. Nothing bad happened, apart from a bit of a stalker and some trolling. There were a couple of things I said I shouldn’t have. But of course I don’t want people to not like me, which is pathetic.

“The thing is I don’t see myself as judgmental and I don’t see any way round social media other than just backing off. I rarely use Twitter; I still post on Facebook, where I've a lot of writer and publisher friends so it’s sort of work. I did look up certain well-publicised situations for this book, then felt bad about that. Now, I write posts, then delete because I’ve become really nervous about the whole social media thing. I'm the sort of person who doesn’t think before she speaks. My husband often says, ‘Hey, take that thing offline.’”

Now that her daughter has left home to study at St Andrews University, FitzGerald says: “I am coming to terms with the empty nest syndrome, although we still have Joe, of course. The mother-daughter thing has, though, been occupying my thoughts a lot lately. If my daughter were in Su-Jin’s situation, like her mother in the book, I would go on the rampage. All the time I was writing Viral I had this sign above my desk saying, ‘Do not kill anyone. This is not about a murder,’ despite the fact that I really like killing people in my novels.” Whether she did so or not in Viral is for readers to discover.

Finally, I ask whether anything she can create in fiction can ever compare with fact. Over the years, she has worked with many sex offenders and therefore knows more than most people about the evil that men do.

“Nothing I could make up would ever compare to reality. The first sentences people have said to me! Obviously, I can’t use any of that, but if I did, people would never believe it. I like to put my characters through hell but I have never put anyone in a book through as much hell as people I’ve worked with have gone through. But I guess I’ve just always been interested in what makes people do bad things.”

Viral by Helen FitzGerald (Faber & Faber, £12.99).