TOM and Giovanna Fletcher are known for their upbeat writing – he for his songwriting skills with bands McFly and McBusted and his children's books, she for her romantic fiction and blogs about being a mum.
Now, the husband and wife team have collaborated on a much darker project: Eve Of Man is their first novel together, a dystopian tale set in a bleak futuristic world which man has all but destroyed with the population at ground-level battling to cope with a devastated landscape swamped by horrendous flooding beneath a hazy sky of pollution.
Inexplicably, no females have been born for years, apart from 16-year-old Eve, who is apparently the only hope of saving humanity.
She lives in the Dome at the top of the sky-high Tower, tended to by the Mothers in a world inhabited by hologram humans, and at the mercy of a frightening organisation.
Three potential males have been selected for Eve – to help her procreate and save the world – but she falls in love with Bram, not one of the chosen few, and sets out to take back control of her life and find freedom.
It's essentially a 'young adult'-orientated love story set against a backdrop of devastation and the Fletchers have made a canny crossover, offering existing fans something different while hopefully attracting new ones in the process.
The idea of a society with no females was sparked five years ago, when Giovanna (33) was chatting to a friend who'd just had a baby.
"She was just saying how everyone in her street had all had boys," explains the writer, who is expecting the couple's third child in September.
"I said to Tom, 'Is that nature? What happens if that happens all the time?'"
Tom (32) continues: "It was one of those, 'What if?', conversations. What if all babies born are boys, for a week, a month, or longer? Would we be looking at the extinction of the human race?'"
The novel echoes their real concerns about the way the world is going – global warming and the damage we are causing to the planet – and their fears for their own children's future.
"You go through moments of being depressed, especially as parents. It affects you more when you read the news stories," says Tom.
"We've done a lot of research and reading on whether we could have a mass flood in London and what the flood levels could rise to, and you realise this stuff could actually happen and potentially it's not that far away.
"There are some great websites where you can look at future predictions of global warming or the fallouts of war and the technologies that would be developed because of those things.
"I wanted to make sure anything we were talking about could have roots in what's happening now. It was depressing to write at times."
Yet, meeting the couple today, they seem far from harbingers of doom. From their comfortable body language and relaxed, funny banter, it seems the childhood sweethearts remain blissfully happy.
They've known each other since they were 13, having met at the Sylvia Young Theatre School. Since marrying in 2012, they've had two boys – Buzz (4) and Buddy (2) – with another baby on the way, although they've chosen not to find out the gender.
Giovanna admits writing about a world devoid of females has made her wonder if she'll have another boy.
"I kind of feel I'd be safe having a boy, because I know what to do with them," she says with a laugh.
"I would like a girl, but no more than I'd like a boy. You're just adding another personality to the mix."
They both admit they've had meltdown moments in their careers, when they felt they'd taken on too much and found it difficult to find a balance, but the strength of their relationship has seen them through.
"We make each other laugh - that's a massive thing and when we are pains in the butt, the other one doesn't take it too personally," says Giovanna.
"That's what you think," Tom retorts.
Giovanna's due date in September provides them with the very physical deadline of the second book in the trilogy, Tom observes.
Both are hoping for a movie deal, and Tom reveals there have already been conversations in that area but remains tight-lipped on further details.
"I like the idea of developing stuff ourselves, rather than handing it over to other people, not that I'm a control freak!," he says"
"I love film. Millie Bobby Brown could pay Eve. Tom Holland would be really cool as Bram."
The pair have two separate offices in their home in Northwood, Middlesex, to focus on their writing.
Alongside the deadline for the second book in the trilogy, Tom's next children's picture book, written with McFly bandmate Dougie Poynter, is also out in September – so there won't be much paternity leave, he says.
The couple are still pondering names for the new baby, but if they do stick to the 'B' theme, they won't be following Tom's mum's suggestion:
"My mum suggested 'Button', but we're definitely not using that," he chuckles.
"It would be 'Butt' for short, wouldn't it?"
:: Eve Of Man by Giovanna & Tom Fletcher, is published by Michael Joseph, priced £12.99
