This season of Once Upon A Time has gone topsy-turvy. The once-savior of Storybrooke, Emma (Jennifer Morrison), is now the Dark One, and one-time villain Regina (Lana Parrilla) has become so light now that when she needed to open a portal to Camelot in last week's episode, her magic wasn't dark enough to do so.

Fortunately, Zelena stepped in and inadvertently helped the Storybrooke residents make their way to Camelot, where Emma has already begun the struggle between being good and evil.

Of course, if last week's closing scene is any indication, Emma lost that fight and somehow embraced her dark side. And once more, the residents of Storybrooke are unclear on how that happened because their memories got erased ... again.

A sneak peek at this weekend's episode, though, unveils another mystery: that of the missing Merlin, the sorcerer who could help Emma escape the darkness. However, in this clip, the knights of Camelot explain to the Storybrooke residents that the greatest magic-user of all time is trapped inside a tree.

Perhaps the most important part of this clip shows how upside down the Once Upon A Time world is now: Emma is no longer the savior. So it's up to someone else to step up and take on that role.

And the person who does that is the least likely candidate: Regina, once known as the evil queen, the evil sorceress who cursed the Enchanted Forest, trapped its people in a world without magic and killed and maimed a lot of others who stood in the way of her own selfish goals.

But over the course of three seasons, Regina, who also serves as mayor of Storybrooke, redeemed herself, and now she's the hero Storybrooke needs.

"It's one thing to ask everyone's forgiveness; it's another thing to lead people," said Once Upon A Time executive producer Edward Kitsis to EnStarz. "So we've seen Regina on our side helping out the heroes, but she's going to be thrust into a leadership position in saving the town, a town where, quite honestly, a lot of people are still frightened of her."

Of course, that puts Regina at odds with the other "heroes" in town, especially Snow White, who's used to being the hero herself.

"That also impacts Regina's relationship with Snow, who's someone who has been a leader and people have turned to," said Once Upon A Time executive producer Adam Horowitz. "It becomes, hopefully, very complicated and messy in a good way, which is these kind of familial and adversarial connections are now forced to interact in new ways."

Once Upon A Time airs Sundays on ABC.

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