Outlander, now in its fourth season, captured our hearts from the moment it aired in 2014. But what is it about Claire Randall and Jamie Fraser's time-travelling love story that has us so enthralled?
It could be the casting of two leads who seemingly have won the genetic lottery, or it could be that the chemistry between them is strong enough to radiate through our screens.
While both are equally important, the main reason for Outlander's success comes down to something more.
Unlike typically clichéd dramas, the series doesn't centre on romance. Instead, it's about love, that real, intangible thing − and the magnetism that draws together two very different people from two very different worlds.
Claire and Jamie conquer time and space itself in the name of love, a grand-scale achievement that tugs at our heartstrings.
The 200-year gap in their timelines is an undeniable and overarching issue the lovers must face. But the aspect of time travel is also what makes their affair so unique.
Outlander was almost guaranteed a huge fan base thanks to the success of its source material, Diana Gabaldon's bestselling novels.
However, the response to the screen adaptation by Ronald D Moore (Battlestar Galactica) – and the casting of Caitriona Balfe as Claire and Sam Heughan as Jamie in particular – has been overwhelming.
"Our fans are the most passionate, and I think a big part of that is that this book series existed for so long," Caitriona, 39, tells TV WEEK Close Up.
"Diana created this incredible world and we've been so lucky that the show has this fandom and we've been embraced by it."
Adapting such an adored series can be tricky, but Sam and Caitriona have brought their beloved characters to life, and as true to the novels as possible.
"We always read the books before, and I know Caitriona and I will look over them again," Sam, 38, explains.
"Countless times when we go and look in the book, Diana's written it perfectly."
Leading US television critic Ed Martin is surprised that the two leads have not been candidates for Emmy awards.
"Given the intensity, passion and intimacy of their performances, it's inconceivable that Caitriona and Sam have never been honoured with Emmy nominations," he says.
"Like Game Of Thrones, Outlander has shown us that well-done adaptations of popular novel series can make for first-rate television."
When we first meet Claire, she's on her second honeymoon with husband Frank (Tobias Menzies) after the end of World War II.
But a trip to the stones at Craigh na Dun in Inverness, Scotland, transports the English nurse to 1743.
There, she encounters Highlander Jamie shortly after being attacked by British military captain Black Jack Randall (also Tobias Menzies), an ancestor of Frank.Claire quickly warms to Jamie's kindness and protective nature, which also captures the hearts of viewers.
As the sexual tension simmers, the pair are tied together by a twist of fate and forcibly married. It doesn't take long for the newlyweds to give in to their true feelings.
They endure witch trials, run-ins with the British and, of course, Black Jack's relentless vendetta. And while their love deepens, Claire finds herself torn between her two lives − and her two husbands.
Love prevails and Claire chooses Jamie. When she discovers she's pregnant, she's persuaded to go back to 1945, where she can raise the child safely.
Jamie, however, remains in 18th century Scotland and heads to fight at the Battle Of Culloden.
Torn apart by time, Claire believes Jamie has died in the battle and tries to move on, bringing up daughter Brianna (Sophie Skelton) with Frank. It takes Claire 20 years to discover her true love didn't perish, and once again she braves the stones to head back in time.
While Claire manages to find her long-lost love, they struggle to reconnect after many years apart and different life choices.
Now, at the start of season four, Claire and Jamie are washed ashore in America after having survived a shipwreck. The pair set out to start afresh.
"We finally sort of get to see them together and settling down and being sort of domestic, almost," Sam says.
It's no secret Outlander has become famous for its passionate sex scenes, which have catapulted Sam to sex symbol status – something the star remains modest about.
"I'm still surprised when people come up and they're really excited," he says.
"It's about the character. Jamie Fraser is like this unicorn of men and that's down to Diana Gabaldon and her writing. So I'm lucky to play that character."
As actors often say, filming a sex scene is just part of the job. For Sam and Caitriona, it's no different.
But having been complete strangers before coming into the show, the two agree that throwing themselves into their raunchy scenes quickly cemented their friendship.
"When we first met, we had to dive into these really, tough and intimate scenes," Sam recalls.
"That's made our relationship a lot deeper and we feel comfortable with each other now. It really bonds you."
Those countless hours on set portraying a couple has meant Sam and Caitriona are nearly as in tune as the real thing.
"It's like a marriage, almost," Sam explains.
"I think we both know each other's tells. I certainly know when she's frustrated or tired. I think we both need a lot of caffeine in the morning!"
Caitriona enthuses, "Sam and I are such good friends. We can talk about how scenes are going to go, whether they're love scenes or fights.
We understand where each other is at and we're able to have a shorthand that really helps."
"It's been a real journey for the two of us," Sam says of his time with Caitriona.
"I feel like she's one of my best friends and it's lovely."
Outlander airs on FOX Showcase.