Lana Parrilla, Jamie Dornan

Once Upon a Time is heading back to the past!

On Sunday's episode, the ABC fantasy drama will flash back to the first week of the evil curse went into effect and ripped our favorite fairy tale characters from their world and sent them to Storybrooke. What will we learn from the past? And what's in store for the future? The cast dishes on what's ahead:

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Regina's happily ever after: Though it's an evil curse, to Regina (Lana Parrilla), it's the best thing that's ever happened to her. "This is her happy ending and that's how she really sees Storybrooke," Parrilla says of the Evil Queen being transported to this new realm. "We quickly learn that it's not. Deep down, Regina just wants to get back to being that sweet, innocent little girl who fell in love, and it's all just been taken away from her." This will shed some insight into why she decided to adopt Henry (Jared Gilmore). "She sees an opportunity to have that healthy love with Henry," Parrilla adds.

Sheriff Graham returns: Thanks to the show flashing back, Jamie Dornan will be back as the now-deceased sheriff and former flame of Regina. "We get to see his relationship with Regina and how she is in control of him and manipulating him by keeping his heart in that box," Dornan says. "She's using him as a puppet and getting him to do whatever she wants. You see Regina at her most evil in getting things." Graham will have his hands full when two "crucial" strangers (see more below) find their way into Storybrooke.

Regina's revenge: Thanks to Snow White (Ginnifer Goodwin), Regina has now lost her first love in Daniel (Noah Bean) and her mother Cora (Barbara Hershey) — so the Charming Family better watch out! "All bets are off," Parrilla says. "When someone is taken away from you like that, something snaps in a person. I think she's more dangerous than she's ever been. She's so angry." Parrilla adds that she'll also set her sights on Rumplestiltskin (Robert Carlyle), despite his insistence that he wasn't involved.

Tough love: Following Cora's death, Snow White will find herself putting up walls, even towards her own family. "You have to love yourself in order for someone else to be able to love you," Goodwin says. Fortunately, her daughter Emma (Jennifer Morrison) will know exactly how to get her out of this stupor. "Obviously everyone is being delicate with her to a certain degree, but Emma is being a little tough love with her and going, 'You know what? At some point, you have to accept what's happened and then we'll deal with it, but you can't sit around regretting it,'" Morrison says.

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Henry's in danger: After a seer told Rumplestiltskin that the young boy who would lead him to his son would ultimately be his downfall, Rumple set his sights on Henry, suspecting that he's the boy from the prophecy. Unfortunately, no one will actually find out about Henry's fate anytime soon. "There's this really serious creepy undertone to what would normally be normal conversations," Morrison says. "But the audience will know Rumple is contemplating what to do about Henry, whereas everyone in the room thinks Henry is safe, so it heightens that sense of danger for Henry because there's no one looking out for him yet because no one knows he's in danger."

Rumple's own issues: Although Rumple will have to watch out for Regina, he'll have problems closer to his heart to deal with. "He's got his son and his relationship with Belle [Emilie de Ravin] to think about, so he's distracted while all this is going on," Carlyle says. "What he thinks about that isn't really clear until Episode 22, at the very end, where you realize what he's actually up to. It's another game-changer of an episode. Everything turns around in that moment." Carlyle also notes that the end of that same episode "is massive in terms of the relationship with Rumple and his son, and it can never ever be the same again."

Star-crossed lovers? Now that Neal (Michael Raymond-James) is back in town, Emma will have to set her feelings about him aside for the sake of Henry. "She's not in a mind space of, 'Oh gosh, I wish that he was with me right now!'" Morrison says. "There's too much for them to work out right now. He let her go to prison and let all these bad things happen to her, and he hasn't been given an opportunity to explain that in a way that really makes that OK." Had Neal gone to prison in her place, she would've waited for him and it would've messed up the curse, so "we'll see if she has the forgiveness in her," Morrison says of an upcoming episode that addresses his misdeeds.

Hook's revenge: Rumplestiltskin is still alive, which is both good and bad news because Hook (Colin O'Donoghue) has spent the last 300 years or so attempting to get his revenge on him. "I don't think he's going to be too happy about it," O'Donoghue says and adds that Hook will still continue his quest for vengeance in a very interesting way once he returns to Storybrooke.

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The stranger: In Sunday's flashback episode, it's unclear whether the two strangers who make their way to Storybrooke in the past will have a connection to the new stranger (Ethan Embry) in town, but Josh Dallas says the residents of Storybrooke will be desperate to uncover the truth. "They're going to keep trying to unearth who this guy is," Dallas says. Though he's not the only one they'll have to worry about since Neal's fiancée Tamara (Sonequa Martin-Green) is coming to town. "Now we have Neal's fiancée trying to accept fairy tale characters in real life," Morrison says. "It's a complicated dance of trying to protect everyone and do the right thing."

Belle's flashback: Get ready to meet Lacey, Belle's alter ego who we know may be drafted by Regina. Ruh-roh. But in the Belle-centric episode, where she'll finally gets out of the hospital, we'll get a flashback to the early days of the RumBelle relationship and how it developed, says de Ravin, who notes that the infamous gold dress will return.

RumBelle hopes: Speaking of, after Rumple professed his love for Belle on his deathbed, he'll have some explaining to do in the coming episodes. "He visits her in the hospital and apologizes for the phone call because it must've freaked her out. But she says she's glad he's still alive and she could tell that the feelings were real," Carlyle teases He also adds that Belle doesn't necessarily remember him, but she has a feeling they may have had a past. "That's the beginning of something else, but in typical Once Upon a Time fashion, it goes a different way."

Going home: With Snow White unsure whether Emma would go back to fairy tale land, it's up to Charming to try to get his whole family home. "It's his home and it's where his soul is," Dallas says. "The fact that he could have all his family there together is even better. He's got a plan that's underway right now that will hopefully get them back at some point. He's definitely going to try to convince as many of his family members as possible to come with him."

Once Upon a Time airs Sundays at 8/7c on ABC.