Forget the hoofers of “Smash,” the dinosaurs of “Terra Nova” or the post-Charlie Sheen refurbishing of “Two and a Half Men.” The biggest long-shot of the 2011-12 season was ABC’s foray into fantasy with the Evil Queen, Snow White, Rumple-stiltskin, Pinocchio and the rest of the gang in “Once Upon a Time.”

The ABC Studios show, created and exec produced by “Lost” alums Edward Kitsis and Adam Horowitz, is an intricate construction of two parallel worlds — fairy-tale land and the contempo cursed town of Storybrooke, Maine — with core characters who inhabit different personas in each realm as storylines drawn from age-old fables converge. It’s a multilevel chess game for producers, further complicated by elaborate vfx, makeup and costuming needs, and it puts unusual demands on cast members Lana Parrilla, Jennifer Morrison, Robert Carlyle, Ginnifer Goodwin and 11-year-old Jared Gilmore.

Appropriately, the fairy-tale drama is delivering a happy ending for the network as its first season wraps May 13. “Once Upon the Time” has defied the odds against fantasy fare on broadcast TV to become a big-tent Sunday night success story. It ranks as ABC’s No. 3 show in the adults 18-49 demo, behind only “Modern Family” and “Grey’s Anatomy.”

“Once’s” viewer base is buoyed by its appeal to two distinct demos: genre fans and family auds. It ranks No. 2 among all scripted primetime skeins (behind only ABC sitcom “The Middle”) among programs that adults and youths watch together. The show clicked with viewers out of the gate in its October premiere, and has held steady throughout the season in its 8 p.m. berth, with an average of 11.6 million viewers and 4.0 rating/10 share in adults 18-49, according to Nielsen.

ABC has had a prosperous year with its freshman shows, fielding bona fide hits with the soapy drama “Revenge” and domestic comedy “Suburgatory.” But none of the Alphabet’s frosh crop was as much of a convention-defying gamble as “Once,” so the rewards are particularly gratifying. The show reps a big investment for Disney on many levels, not the least of which is allowing producers to work with a handful of characters, such as Jiminy Cricket, that have a strong association with the Disney brand.

“This show has very, very complex production issues,” says Barry Jossen, exec veep of ABC Studios. “But maybe the most complicated thing of all was taking the creative leap of faith that a TV series starring beloved fairy-tale characters could be a broadly appealing primetime series.”

That the series is so well-realized is testament to the skill of Kitsis and Horowitz, and the fact the longtime writing partners nurtured their idea for the show for nearly a decade. The central premise revolves around answering the questions about what happened to Snow White, the Evil Queen, et al, in the periods surrounding their oft-told tales, e.g. before “once upon a time” and after “happily ever after.”

“We’d always talked about how much we loved these stories and how formative they were for us,” Horowitz says. “And we always talked about how hard it would be to be the Evil Queen in a land of happy endings,” Kitsis says. “We liked the idea that these stories could be happy and sad, dark and light.”

It took years to bring “Once” to fruition, because the two knew they didn’t have the chops to pull off such an elaborate production when the idea was first conceived, back when they were staff writers on the WB’s “Felicity.” But after six seasons on “Lost,” they’d earned their showrunner wings. ABC came to Kitsis and Horowitz with an offer to develop a different project. The duo passed on that idea, but talked up their fairy-tale fantasy. Jossen listened intently to the pitch and responded: “Sunday at 8.”

ABC Entertainment prexy Paul Lee got behind “Once,” giving it extra marketing TLC even as ABC had a slew of new shows to support last fall. The Alphabet also made the wise decision to hold its preem until Oct. 23, well past the September crush of new and returning show debuts. That had the effect of making “Once” stand out from the pack, and it gave ABC more breathing room to sell the unusual concept to viewers.

For Kitsis and Horowitz, the riskiest part of the process was casting the pilot. The high-concept story left no room for so-so performances. The audience had to be sold on the actors playing multiple characters, talking to tiny fairies, communicating with wolves, casting spells, sparring with dwarfs, among other fantastical adventures.

No role was more crucial than the Evil Queen, who bedevils Snow White and puts a curse on Storybrooke, where she exists as icy single mom and mayor Regina Mills. The casting anxiety eased up early on after Parrilla auditioned. “She walked in, and we looked up and said, ‘This is our queen,’?” Kitsis says.

Horowitz adds that the goal in writing the characters was to turn them into flesh-and-blood people, and the cast has managed to find the humanity at their core.

Many of the characters invoked in “Once” are centuries-old, but producers have to work with Disney’s brand management department whenever they involve Disney-fied properties, like the Seven Dwarfs with the characteristics conferred on them by the 1937 animated feature.

But the extra layer of scrutiny hasn’t been an obstacle — far from it.

“Not only did they let us add a dwarf, but they let us kill one,” Kitsis says. (The heretofore unknown eighth dwarf, Stealthy, was added to Snow White’s entourage in episode 10, “7:15 a.m.,” and was killed in a jailbreak involving Grumpy.)

The demanding production process on “Once” has spurred ABC Studios to pursue innovative approaches like real-time collaboration on vfx through streaming video. Directors do much of their prep work through iPad tools that allow them to do a 360-degree tour of all the show’s virtual sets, plan shot lists, select lenses and lighting schemes all in a fraction of the time such advance pre-production would normally take. Jossen boasts that every one of “Once’s” 22 episodes came in on time and on budget, a feat aided by the contributions of exec producer Steven Pearlman and producer Kathy Gilroy.

Even as they begin their prep work for season two, Kitsis and Horowitz are still coming to grips with their good fortune on their first time out as series creators.

“I never thought we’d get past Act 2 in the pilot,” Horowitz jokes.

“Every once in a while we take a step back and look at a scene like Geppetto talking to a fairy and a cricket, and there’s really intense drama there,” Kitsis says. “(Then) there’s part of me that feels it’s ridiculous that it’s even on the air.”