"Everybody thinks Zelena's in that jail cell. They're going to find out very quickly that she's not," executive producer Adam Horowitz tells THR as he and Eddy Kitsis set the stage for Sunday's two-hour closer.

An unwanted trip to the past is in store for the season-three finale of Once Upon a Time.

Rumplestiltskin broke his promise to his fiancee, Belle, with a bit of trickery and sleight of hand, killing Zelena with the dagger and watching as her body shattered into pieces in the Storybrooke jail cell Regina put her in. As Zelena's body disappeared into thin air, unbeknownst to Emma and co., a time portal opened even without Zelena present. In Sunday's two-hour closer, "Snow Drifts" and "No Place Like Home," Emma and Hook find themselves stuck in the Enchanted Forest of the past with any change threatening their existence and those around them.

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"We really envisioned this to be a two-hour movie. It straddles the line between [being] self-contained and what we think is a satisfying climax to the year. All the questions that we've posed during the year will get answered in these final two hours," executive producer Eddy Kitsis tells The Hollywood Reporter.

Ahead of Sunday's finale, Kitsis and Adam Horowitz chat with THR about the aftermath of Zelena's defeat, the time portal to the past, Emma and Hook's "difficult" trek back to Storybrooke and hints on a possible new destination.

We've seen Emma and the gang go up against a number of foes over the past few seasons. How would you rate Zelena against the rest?

Edward Kitsis: She is the most wicked. What makes her formidable is that you could see that she is just this prodigy of magic. You just see just how super talented she is. But what makes her so formidable is that she has so much pain behind her, and torture.

Adam Horowitz: We've been exploring many spokes of the wheel of evil and they each have their different personalities. Peter Pan had his own conniving, manipulative ways. And Cora with her familial issues that fueled her rage. Now with the Wicked Witch, we've come at her from a slightly different angle. It's all been fun in their own ways. We've been really enjoying Rebecca [Mader].

Kitsis: Rebecca has made her so formidable. We wrote the part for Rebecca. We knew we wanted to bring the Wicked Witch in this year and we knew it was Rebecca. We literally just called her and said "This is what we want to do" and she knocked it out of the park.

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Is there another spoke in the wheel of evil you want to explore in the future?

Kitsis: One of the biggest evils we all have is the one inside us. That is always a spoke.

Horowitz: We said it all the way back in season one. Evil isn't born, it's made, and there's always room on the wheel for another spoke. We may be seeing more of that.

Kitsis: And the question of what is evil? There's been lots of things presented to us as evil that we found have just been misunderstood.

It's been interesting to watch the "villains" becoming heroes, the most recent example being Regina. Is that dark element a chapter that's closed for them?

Horowitz: I don't think anyone has a chapter in their life they're ever completely done with. You overcome and get past things but they're always a part of you and they always make up who you are. Just in the last episode, where we saw Regina use light magic, that doesn't mean there's still darkness that's a part of her that she's done with. We saw Rumple, even with his darker impulses, doesn't mean that there isn't good in him as well.

Kitsis: Rumple promised Zelena he wasn't going to kill her, but at the same time when he proposed to Belle, that was genuine.

Let's talk about Regina's decision to spare Zelena when she locked her up in the cell. Will Regina regret not killing her when she had the chance?

Kitsis: The two hours will absolutely answer whether she regrets that decision. (Laughs.) And it will become very, very obvious.

Had Rumple not come in and killed Zelena, do you think she had a good shot at redeeming herself like Regina had hoped?

Kitsis: I think this is a show that when Adam and I created it, we wanted to say everyone had a shot at redemption. It was about hope. If Rumplestiltskin, the most selfish man in the world and someone who was a coward, can literally sacrifice his life and break the cycle of his family's cowardice, which we saw in episode 11, I think everybody is capable of it. It's really a matter of whether you choose to do the hard work to get there.

What does a Belle and Rumple engagement look like? He's hiding a big secret from her, and we know secrets never end well in Storybrooke.

Kitsis: Secrets never end well. (Joking.) I can tell you that his bachelor party is going to be in Atlantic City.… As we said, Rumple's love for Belle is genuine but he also let us know in their very first episode together, "Skin Deep," that he's a very difficult man to love. There are a lot of complicated things going on in his head and we're just beginning to explore that. Next year, we'll understand a lot more what he's thinking.

Will Belle find out about Rumple killing Zelena?

Kitsis: We'll have to wait and see when she finds out. It could be this weekend, it could be next year. You'll have to watch Sunday night.

Horowitz: The other question related to that to ask is everybody thinks Zelena's in that jail cell. They're going to find out very quickly that she's not. What's that going to mean? That comes to a head pretty quickly this weekend.

At the moment, Emma is set on returning to New York to resume her life with Henry, but as we tee up the finale, Emma and Hook find themselves in a precarious situation: the Enchanted Forest of the past. How do they find themselves there?

Kitsis: We know that Zelena had a time travel spell, a time portal or a time hole. But last week we saw it enacted even though she was defeated. To me that being open is a bad sign for our residents of Storybrooke. This year, we started out with Emma in Neverland being a leader getting her son back, but we find out emotionally that she still felt like an orphan and right now, her greatest memories are fake ones of being in New York. Emma is a character who's looking for home, but because she's never had one, she doesn't know what it's like. That is why the finale is called "No Place Like Home," because we are going to have Emma ask that very question. And fitting during our Oz-themed run.

Hook wasn't happy when Emma said she was thinking of leaving town. What is he going to do to ensure that that doesn't happen?

Kitsis: He is getting fed up because it's like the entire world cannot resist my charms, why is one person I fall in love with so resistant to me. He is becoming frustrated because he has declared that he will win her heart and she has declared it's going to be very hard. They are going to be put in a lot of danger this week and we'll see if romance can even rear its head.… But I'd be depressed if it didn't.

There's a glimpse of Emma and Hook in period garb in the promo. Any hints you can share?

Horowitz: They're cosplaying at Comic-Con.

Kitsis: Yeah, they're either cosplaying at Comic-Con or they're up to some shenanigans.

Horowitz: I think it may be shenanigans.

Kitsis: Sometimes you have to go undercover. Sometimes it's just fun.

How difficult will it be for them to get back to Storybrooke?

Kitsis: I hope it's difficult or else it will be a very boring two hours.

Horowitz: There are many challenges if they want to return, not just to their present but to their present the way they remember it.

How much thought went into the naming of Snow and Charming's newborn son?

Kitsis: That will be answered this weekend. When people hear the name, they'll know why and how we came up with it.

It's been teased that the final moments of the finale will reveal where the show will go next. Anything you can divulge?

Kitsis: It is something we cannot wait for, and I hope to God it doesn't get spoiled.

Horowitz: I hope people are as excited about it as we are. It's something we'll look forward to talking to you about after. There's really no way to tease it beforehand without giving it away.

How different will the world be following the events of the finale?

Kitsis: You set up things in the beginning of the season and you hope to have certain things resolved, but I definitely think some of the characters are going to find themselves in happier situations than others.

Should we be worried about significant sacrifices or deaths?

Horowitz: You always have to worry about death.

Kitsis: Every day you wake up you should worry about death. That is what my mother taught me at a very young age.

Looking ahead to season four, have you honed in on how you'll bring Michael Socha's Knave into the fold?

Horowitz: We've had extensive discussions with the writers already …

Kitsis: Mapping out next season.

Horowitz: We're very excited about the possibilities for Michael and all the characters.

Kitsis: He was a member of [Robin Hood's] Merry Men. We think he'll fit in well in the show.

Lastly, how would you describe the final two hours of the season?

Horowitz: Wish fulfillment.

Kitsis: Epic wish fulfillment.

Once Upon a Time wraps up season three with a two-hour finale Sunday at 8 p.m. on ABC.

Email: Philiana.Ng@THR.com

Twitter: @insidethetube