Elizabeth Lail, Georgina Haig

Let it go? They just couldn't. When Once Upon a Time creators Eddy Kitsis and Adam Horowitz went with their families to see Frozen last fall, they so flipped for the Disney animated fantasy that they immediately began brainstorming ways to bring some of its characters to their ABC series. After all, OUAT had already played host to a wide array of Disney favorites, from Snow White and her Seven Dwarfs to Peter Pan, Pinocchio, Maleficent, and Mulan. Why not bring on those wildly popular Frozen sibs, Princess Anna and Queen Elsa, and give them a cool new story in Storybrooke?

Not so fast. Even though Disney owns ABC, it's one thing for the fiercely protective company to allow Kitsis and Horowitz to play with characters that date back several dusty decades. It's quite another to let them dip into the Oscar-winning Frozen a mere 10 months after its release. This white-hot cash cow has earned $1.2 billion at the box office and is still shattering DVD and soundtrack sales records. (It's the top-selling album of the year.) A sequel is highly likely, and a Broadway musical is in the works.

"Frozen is a phenomenon on an entirely new Disney scale," says Kitsis, who noted that the studio's lending of Anna and Elsa at the height of their earning power and pop-culture supremacy is "not the same as letting us borrow 80-year-old Snow White. But that didn't stop us from asking." The execs eagerly sent off their story pitch and waited while it passed through several divisions of the Mouse House — TV, film, animation, corporate brand management. "Pretty much everyone had to weigh in, including Goofy," cracks Horowitz. But approval was unanimous.

The film and the series do seem perfect for a mash-up. Frozen is the story of Anna, an adorably daffy optimist, who goes on an epic trek to reconnect with her sister, Elsa, an emotionally distraught loner whose ability to create ice and snow has left the kingdom of Arendelle in a never-ending winter. "The theme of Frozen is the power of love, and that's been the theme of our series since Season 1," says Kitsis. "Also, we both put out the message that it's OK to be different. In fact, it's better than OK — it's to be ­celebrated. Plus, we rip out hearts. They freeze them."

Though the 11-episode Frozen arc — featuring former Fringe star Georgina Haig as Elsa and fresh-from-college newcomer Elizabeth Lail as Anna — will officially launch with the season premiere, fans got an early glimpse of Elsa in last May's cliffhanger as she emerged out of Rumplestiltskin's urn and into the real-life village of Storybrooke, leaving an ominous trail of ice crystals in her wake. Now she's heading for a meltdown.

"Elsa is a stranger in a very strange land," says Haig. "She knows nothing about cars and electricity and other modern things, and she's encountering all these new people who don't understand her. After all the progress she made in Frozen and her beautiful happy ending, she's suddenly feeling very alone again." As in the film, Elsa's fear will manifest into a massive snow monster, causing panic in Storybrooke.

As that drama gets under way, a parallel plot set in the past in Arendelle finds Anna and her ice-trader beau, Kristoff (Greek's Scott Michael Foster), on the verge of marriage, but the royal event is put on hold when Anna gets bombshell information about her parents, the late king and queen, who perished in a shipwreck.

"Anna is led to believe her parents went on that voyage in search of something that could help ­control Elsa's powers, but she's just not sold on it," says Lail. "She's determined to find out the truth about their mission. She dearly loves Kristoff but knows she can marry him next month or next year. What matters is her sister."

Lail seems to be living a fairy tale of her own. The actress graduated from the University of North ­Carolina just last May and had moved to New York to pursue stage work when she landed the OUAT audition. "I was mentally prepared not to work for a long, long time, so this is all so overwhelming," Lail says. "I went to see Frozen — appropriately, with my sister — and loved it, but never in a million years could I have imagined that I'd ever play Anna. I mean, what are the chances? Now I've seen the film probably 100 times." She's also a huge OUAT fan. "I walk around the set in a starstruck daze," Lail says with a laugh. "I still blush whenever I get around Josh Dallas [Prince Charming] and Colin O'Donoghue [Captain Hook]."

The Australia-born Haig, by contrast, didn't see Frozen until she was up for the part last June — and that was in the middle of her honeymoon. "When the film was in theaters, I was suffering from a severe case of wedding brain — all I could think about were cakes and fascinators and guest lists," says Haig, who got the call to audition when she and her husband, screenwriter Josh Mapleston, were in post-ceremony bliss on a remote island off the coast of Queensland. "I created my own audition tape — with Josh playing Anna — and emailed it to the casting people," Haig says. "I got the job on the last day of the honeymoon."

Great care has been taken to keep what Frozen freaks love most about the sisters, while still trying to move them forward dramatically. Case in point: Anna's quest will take her to OUAT's Enchanted Forest — conveniently Arendelle-adjacent — where she's in for a big shock. "This is a sunny, hopeful woman who always sees the good in people," says Kitsis, "but what happens when she meets Rumplestiltskin [Robert Carlyle], the devil? We'll reveal a whole new side of Anna's personality."

The plot also features Lost vet Elizabeth Mitchell as the Snow Queen, though her exact connection to Anna and Elsa is being kept under wraps. Other Frozen characters on loan include Anna's ex, the conniving Prince Hans (Tyler Jacob Moore), his 12 brothers, and Pabbie the Troll King (voiced by John Rhys-Davies). Even Kristoff's lovable reindeer Sven is on board, played by a real reindeer named Jack. But where's Olaf? Didn't the execs want to build a snowman? Not really. "Olaf is a fantastic character, but he just didn't feel organic to our story," Horowitz says. "We didn't want to shoehorn him just because everybody loves him." But he will get a shout-out.

Others stirring up Storybrooke this season include Will Scarlet (Michael Socha), aka the Knave of Hearts from the short-lived spinoff Once Upon a Time in Wonderland, and the presumed-dead Maid Marian (Christie Laing), who has reunited with her husband, Robin Hood (Sean Maguire), thus leaving Robin's new love, reformed baddie Regina (Lana Parrilla), heartbroken and steaming mad. Tempted to return to the dark side, Regina goes to her Magic Mirror (Giancarlo Esposito) for dastardly assistance. "When it comes to evil, Regina is like a recovering alcoholic," says Horowitz. "She may hold that bottle in her hand, but will she really take a drink?"

And will she ever forgive Emma (Jennifer Morrison) for rescuing Marian and bringing her to Storybrooke? "Emma will not apologize for saving a life, but she's feeling really guilty about getting in the way of Regina's happiness," Morrison says. "Emma does not want to see Regina regress. Who does? It's to everyone's benefit to keep her on the good side of things."

Emma shared a tender kiss with her big crush, Hook, at the end of last season, but don't expect them to set up housekeeping anytime soon. "Our audience gets so mad when we don't let couples relax and be happy," says Kitsis. "We're like, 'Really? You want to see Emma and Hook rent a movie and stay home? Oh, look, they're making popcorn and Hook needs help because he only has one hand! Isn't that adorable!' Where's the drama in that?"

Point taken. But couldn't they have just a wee bit of happily-ever-afterness? Says Horowitz: "We will definitely explore the feelings Emma and Hook have for each other and the reality of these two people trying to get together...while running from a snow monster."

Once Upon a Time returns Sunday, Sept. 28 at 8/7c on ABC.

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