Back in 2008, when scriptwriter Michael Hirst first imagined a television show based on Norse mythology, it was little more than a wishful whimsy, hatched over a few glasses of wine while on a family holiday in France. It's a moment that his daughter, Georgia Hirst, who stars as Torvi, Princess of the Danes, in Vikings, remembers well.

"It was about 10 years ago, when I was 13 or 14, and we were just lazing around on holiday. Suddenly, my dad said: 'I'm going to write a TV show and it's going to be about the Vikings, so if you have any ideas for the opening scene, let me know.' It was weird, we were like, 'What are you on about?'

"So, when it all came to fruition, it was brilliant," she adds, proudly. "But it was also kind of surreal, because I was so young when he'd thought of it. I certainly never envisaged that, one day, I'd be a part of it, like I am now, or that it'd become this massive. That's so crazy."

If she never envisaged Vikings' genesis, nor its runaway success, then neither did her dad: he's clearly still dumbfounded by its meteoric ascent from a low-key, historical drama on the History Channel, in Canada, to becoming the behemoth it is, watched by millions of ardent fans, in over 250 territories, across the globe. Now into its sixth season, it's one of the biggest television shows ever, rivalling Game of Thrones in its success, scope and storylines.

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"It has been a bit of a surprise just how well it's been received," admits Hirst senior, bemusedly downplaying the show's stellar success and fervent following. "I suspected there might be some interest out there, because people have always been fascinated with Viking tales and Norse mythology. But could I have imagined it becoming a big as it has? Well, no. You never know that. It's in the lap of the gods and depends on the audience figures."

Despite his doubts, it didn't stop Hirst dreaming big, aiming high and planning multiple seasons and plot lines for Vikings – just in case, as he explains.

"We were only commissioned for nine episodes to begin with but I definitely had an idea of where I wanted to take this. I was desperate to do the attack on Paris, because I knew how spectacular that would be and I also wanted the Vikings to get to North America," he reveals, with boyish enthusiasm, during a tour of the Vikings' set, including a full-scale mock-up of the settlement of Katterat, located on the outskirts of Dublin.

"But I knew that both of those ambitions would be a long process and require many, many series. Really, it was wishful thinking on my part because more than half of new shows get cancelled almost immediately."

Wishful thinking or not, his fantasy soon became reality as Vikings' twisted, titanic plotlines centred on kinship, family feuds, epic battles and pillage captivated audiences all around the world. In effect, like the Vikings had achieved a thousand years before, one territory after another has been conquered by Vikings, making it the No 1 television show in the world - if you believe the press release's hyperbole.

"There have been a lot of Viking series or movies before, but nothing has ever gone through the roof, like this has," nods Alex Hogh, who plays Ivor the Boneless. "This is so popular all over the world, from Argentina and Brazil to Australia and New Zealand. There's a wave of 'Viking-ness' with Vikings popping up in other forms of media and lots of other shows now having Vikings [in them].

"There are some fans who are so devoted to the show that they've got pictures of our characters tattooed on them," he adds. "One even sent me a photo of my character tattooed on his thigh! I never imagined anyone would want to do that, but they do. That shows just how popular Vikings is."

While the scale of Vikings' success mystifies Hogh and Hirst, it shouldn't be that much of a surprise. After all, Hirst has history in this arena: he's the writer, director, producer or originator behind a slew of acclaimed historical drama series, notably The Tudors and The Borgias. He also penned the critically acclaimed, award-winning Elizabeth and its follow-up Elizabeth: The Golden Age, which catapulted Cate Blanchett to superstardom and made historically based fiction sexy and bankable. A large part of that was his penchant for ensuring historical fact, then adding a sprinkle of fiction to spice things up.

"Yeah, we try to stick to history as much as possible but we also have to be aware that we're creating a TV show, so we'll always go with the most dramatic solution, or dramatic way to do a story," explains Hogh. "It's important to do that, I think."

"I don't like fantasy. I like stories that are rooted in reality and I prefer to write about real people, in real situations but whose stories are fantastical," adds Hirst, assertively. "Also, I have an academic background, so the research part of my job is something I delight in. It's out of that research, reading and contemplation that ideas, characters and storylines start to emerge. That process is very important to me and I enjoy it enormously.

"Really, I'm a storyteller at heart, so although it's very factually based and driven, I use that as a jumping-off point to create stories," he explains, animatedly. "Those embellishments are what makes it interesting, rather than just a dull, dusty history lesson. But I also always ensure that what you see could have happened, even if it didn't. That's important to me and, I think, to the fans."

That dedication, his attention to detail, and believable yet fantastical storylines have earned Hirst plenty of acclaim. It's also rekindled a worldwide fervour for Norse mythology and Viking escapades. Whether it'll last another millennium or not remains to be seen.

LOWDOWN
Who: Director Michael Hirst
What: Vikings, season five (part two)
When: Streaming on Lightbox from today