Fairy tales of the unexpected.
From the creators of Lost, the ambitious series merges the world of fairytales with reality in a fictional American town. The stories unravel across two parallel worlds - the mythical fairytale setting of the Enchanted Forest and the modern day setting of smalltown USA.
As the show flits back and forth between the two worlds, viewers are re-familiarised with fabled characters remembered from childhood storybooks.
In the Enchanted Forest, Carlyle plays the Grimms' creation Rumplestiltskin and in Storybrooke, Maine, he plays the enigmatic Mr Gold.
Rumplestiltskin was no Prince Charming, but Carlyle admits that the dual roles of the fairytale character and his real-life alter-ego Mr Gold presented a double whammy of challenges.
He said: "I suppose it's like a jigsaw puzzle. Something you have to try to piece together.
"I think most actors would say the same thing. These are more interesting parts to play, the characters who the plot is kind of bouncing off. It's not quite as exciting when you're pushing the plot through.
"People come and ask me to do these parts because they think I can do that."
Carlyle, who shot to fame after films like The Full Monty, Priest, Trainspotting and Angela's Ashes, was drawn by the fact that the storyline in the grand-scale American series merged fantasy with reality.
He said: "Rumplestiltskin and Mr Gold are obviously intrinsically linked.
"In some way, they are pretty much spawn of the same person. What interested me about the change between the fairytale world and the real world was that there wasn't such a massive jump in actual fact.
"It wasn't as if fantasy fairytale jumped into social realism or something like that. I don't think that would necessarily work. I think Storybrooke [the modern day 'real' town] is slightly off balance.
"Everything's off-kilter. So Gold and Rumplestiltskin, though physically different, in their head there's an awful lot of similarities there. And I think through the episodes that the guys write, we'll begin to discover that."
In the first series, Regina unleashed the Dark Curse, causing the inhabitants of Fairy Tale Land to become trapped in Storybrooke - a place where time is frozen and where the residents do not remember the past.
The spectacular finale saw the breaking of the curse, and now season two deals with the aftermath.
It opens with Mr Gold's potion, intended to restore things that are lost, bringing the magic of the Enchanted Forest into Storybrooke. As a result, the residents are still trapped in our world. They remember their past identities and there are many joyful reunions, but living under the curse has changed them all.
Carlyle has also spoken about getting the chance to "redefine" one of the fairytale world's most famous fictional characters.
He said: "This is the chance for me to define or redefine this character for a whole new generation, at the ripe old age of 51. Everyone knows the name, but that's pretty much all we know. He's in everyone's mind and everyone's imagination; we all know this name from our childhood. But who is he? What is he about? "It takes an awful lot of preparation for every one of these scenes, but as soon as I'm on, I love it and I don't want it to end."
He has even gone on record in the past as saying the character has provided him with "the best 42 minutes of TV I've ever been involved in".
"You see the sadness in the man, you can see the fact that he's intensely lonely," he said.
Carlyle's co-star Ginnifer Goodwin, best known for her appearances in Utah-set bigamy drama Big Love and Johnny Cash film Walk The Line, admitted she found playing two characters a major challenge.
She plays the dual roles of Snow White and Mary Margaret Blanchard.
She said: "It's incredibly inspiring and certainly contributed to my wanting to be a part of it, and I think that it's really also a unique situation.
"What was really exciting was creating a character in Mary Margaret that is based on what we assume are the characteristics the Evil Queen would have wanted to put upon this woman.
"And that was sort of an unusual and exciting process, to take other characters' motivations into consideration in how you express a character."
Co-star Lana Parrilla agreed. She said: "I think anytime an actor is handed a script where you get to play two roles is pretty awesome. I play the Evil Queen, and I also play Regina.
"And I've worked pretty hard at showing the contrast between the two characters.
"The queen is very powerful and puts everything out there, where Regina, she masks everything. I think she's a much more complex character, but I have a blast in both roles. I love them both. It's fun."
And for the writers, the series gave them the head start of having characters already established.
Edward Kitsis, who helped pen smash hit Lost, said: "The show at its core is a character show. We are much more interested in the character than the mythology.
"We are much more interested in 'why does the Evil Queen hate Snow White? Why is Grumpy grumpy? Why does Geppetto want a boy so badly he made one out of wood?' that we love the idea of going back and forth and kind of informing what the character is missing in their life, and that's what going back and forth does for us."
Co-creator Adam Horowitz (One Tree Hill) added: "It's one of the reasons we want to go back and forth between the worlds - we can start the stories at any point we want to and tell any part of them we want to and kind of orient the audience in any way.
"So if you don't know the story of Rumplestiltskin or you don't know the story of Jiminy Cricket, we're coming in at a different place and hopefully, if we do our job right, you will be able to enter and figure out what's going on," he said.
Once Upon a Time, Sunday, Channel 5, 9pm MEMORABLE ... Robert as Scottish bus driver George Lennox in Carla's Song, top, as jobless Gaz in The Full Monty and hardman Begbie, left, in Trainspotting.
Right, he plays Rumplestiltskin in Once Upon a Time.
MEMORABLE ... Robert as Scottish bus driver George Lennox in Carla's Song, top, as jobless Gaz in The Full Monty and hardman Begbie, left, in Trainspotting. Right, he plays Rumplestiltskin in Once Upon a Time.