Jennifer Morrison stars in “Once Upon a Time”

All is not well in Storybrooke, Maine. A hero has become a villain, and it looks as if someone is about to lose their life. Or are they? Fans who want to know have to tune in to the midseason finale of "Once Upon a Time," airing Sunday, Dec. 6, on ABC.

Set simultaneously in a small Maine town and an enchanted realm, the fantasy drama follows familiar fairy-tale characters who also exist in present day. The series began when Emma Swan (Jennifer Morrison, "House") was reunited with the son (Jared Gilmore, "Mad Men") she gave up for adoption. He led her to Storybrooke, where she learned she's really the daughter of Snow White/Mary Margaret (Ginnifer Goodwin, "Big Love") and Prince Charming/David (Josh Dallas, "Thor," 2011), and that she held the key to breaking a powerful curse cast by the Evil Queen/Regina (Lana Parrilla, "24").

Mixing fairy-tale characters with the modern world was a big gamble, but the show was a hit when it first premiered in October 2011, despite many predictions that it would be the first series canceled that season. Now in its fifth season, the series is still chugging along -- "Once Upon a Time" has had its ups and downs when it comes to ratings, but the network knows the show has millions of fans who are as loyal as they come.

The premise is an unusual one for prime-time network television, but co-creator and executive producer Edward Kitsis offered some insight into why it has connected with viewers when he spoke at a panel at this year's San Diego Comic-Con.

"I think that this is a show for believers and people who want to believe, and I think that it united a lot of people that were tired of cynicism and believe in magic," he said. "I think the reason that we created this show was to add a little hope to the world in any way we could."

It was Kitsis' fellow creator and executive producer, Adam Horowitz, who sparked a fan frenzy in October when he tweeted a picture of the cover of the season 5 midseason finale's script, which showed the episode's title: "Swan Song." For those not in the know (spoiler alert!), Emma Swan was transformed into The Dark One at the end of season 4. Fans and critics have been speculating about whose "swan song" the episode features, especially since the producers have promised more than one death this season.

Many are likely hoping the "swan song" in question brings Emma back to the light, though Morrison said at Comic-Con that playing The Dark One has been exciting. "There is some fun to the villainy," she admitted, and went on to explain that Emma had been through some tough times in her life and that "she’s going to face the darkness in herself and figure out what that means and how to truly overcome it instead of just repress it."

Colin O'Donoghue as seen in “Once Upon a Time”

Regardless of what happens to Emma, the episode should set the tone for the second act of season 5. The midseason finale is the last opportunity for fans to visit the Storybrooke characters until "Once Upon a Time" returns in March 2016 with its milestone 100th episode. Horowitz and Kitsis have teased that the second half of season 5 will be hell, leading many to believe that the characters may be literally headed to Hades.

Fans can also expect the return of some favorite characters this season, such as Captain Hook (Colin O'Donoghue, "The Clinic"), Red Riding Hood (Meghan Ory, "Vampire High"), Mulan (Jamie Chung, "Big Hero 6," 2014) and Aurora (Sarah Bolger, "In America," 2002). In an interview with The Hollywood Reporter, Horowitz and Kitsis talked about season 5's LGBT storyline, which will revolve around Aurora and Mulan and is set to take place in the second act. "For us, we wanted to tell it no differently than we would with Robin and Regina or Snow and Charming," said Kitsis. "We just wanted to tell a love story. For us, it's a love story like any other and it's something we're excited about."

With characters popping up from just about anywhere -- from Neverland and Sherwood Forest to Arendelle and beyond -- the series has seemingly endless storyline possibilities. Horowitz and Kitsis have some experience in dealing with a show of such scope -- they both worked on the critically acclaimed ABC hit "Lost," for which they and other members of the "Lost" writing team won the 2006 Writers Guild of America Award for Best Dramatic Series.

Their work on "Once Upon a Time" has not gone unnoticed. The series won the 2011 TV Guide Award for Favorite New Series, and has earned a number of People's Choice Award nominations as well as several Emmy nominations.

The show's popularity also prompted Disney-owned Hyperion books to publish a book about the events in the first season, "Reawakened" by Odette Beane. In 2013, a comic book entitled "Once Upon a Time: Shadow of the Queen" hit the stands, and was followed up in 2014 with "Once Upon a Time: Out of the Past." The show spawned a spinoff as well, "Once Upon a Time in Wonderland," featuring the Alice in Wonderland characters. It premiered in October 2013, but it failed to captivate audiences the way its namesake did, and concluded in April 2014.

Luckily for all involved, fans can't seem to get enough of the original "Once Upon a Time." Don't miss the midseason finale airing Sunday, Dec. 6, on ABC.