On the latest episode of "Once Upon a Time," Regina discovered that she has an unexpected connection to her nemesis, the Wicked Witch of the West.

In "Witch Hunt," Regina (Lana Parrilla) saves Robin Hood's (Sean Maguire) son from a flying monkey, and turns the monkey into a harmless, stuffed toy for the boy to play with. Regina then comes to the conclusion that the flying monkeys have come from Oz, meaning the Wicked Witch (Rebecca Mader) has taken over Regina's castle.

She comes up with a plan to go through the tunnels underneath the castle and break the protection spell so an army can be sent in to attack the Wicked Witch of the West.

In Storybrooke one year later, Snow (Ginnifer Goodwin) explains to Emma (Jennifer Morrison) that she has no memory of the last year.

Hook (Colin O'Donoghue) fills in the blanks of her memory, and tells her that after the spell was cast, the group was transported into the Enchanted Forest.

In addition to losing their memories, the curse is also causing some people in Storybrooke to disappear.

Emma decides that the only way to figure out what is going on and get everyone's memories back is by figuring out who took their memories. Emma asks Regina about the curse, and Regina swears that she didn't do it.

Emma then calls a town meeting to figure out the mystery once and for all. But at the meeting, everyone, including Emma, turns on Regina. Regina then causes an earthquake and disappears in a cloud of smoke. But it's clear that it was all part of Emma and Regina's plan so they can buy time to seek out the real culprit. All the while, the Wicked Witch was at the town meeting, wearing a big grin on her face.

Back in the Enchanted Forest, Regina and Robin Hood go through the tunnels underneath the castle. Regina sees that the Wicked Witch was able to open her mother's crypt, which was sealed with blood magic.

In Storybrooke, Regina and Emma announce that they're creating a memory spell to restore everyone's memories of the past year, and to prove that Regina is innocent. They suspect that whoever cast the curse will try to stop Regina.

The Wicked Witch comes forward, but when Regina and Emma corner her, she disappears in a cloud of smoke.

Meanwhile, the Wicked Witch is pretending to be a midwife in Storybrooke so she can have a reason to be near Snow and her unborn child.

Little John is found in Storybrooke with a monkey bite. He then turns into a flying monkey himself, which leads Regina to believe that they are dealing with the Wicked Witch.

In the Enchanted Forest, Regina wants to put a sleeping curse on herself so she doesn't have to deal with the anguish of losing Henry. But the Wicked Witch stops her before she has the chance to put the curse on herself.

The Wicked Witch reveals that she and Regina are half-sisters, and that is why the Wicked Witch was able to break through Regina's blood magic. The Witch is seeking revenge because her mother and Rumplestiltskin (Robert Carlyle) abandoned her, so she was never able to have the life that Regina had.

Meanwhile, in Storybrooke, the Wicked Witch has brought Rumple back to life, and is keeping him locked in a prison cell.

On the next episode, "The Tower," "Zelena plans a surprise for David as he and the others hunt her, while maintaining her prisoner. Meanwhile, a year ago, Charming tries to help Rapunzel overcome her fears and escape the tower where she is imprisoned."

The Little Mermaid will also return in an upcoming episode. Colin O'Donoghue told TVLine, "You'll get to see Hook and Ariel interact a bit, which I think will be fun. They have a 'tricky' sort of relationship, and I think people will like it."

In addition, Belle (Emilie de Ravin) will have a larger role in future episodes. Co-producer Adam Horowitz said that Belle will be a part of "a significant story that is launched by those scenes from the first episode that involves [her and] Neal."

"Belle's role in Storybrooke in the present is going to reflect on what you see us explore in the past," added co-producer Edward Kitsis. "What we loved about the character [of Belle] was she was a bookworm who wanted to actually experience adventure... and this year you're going to start seeing her kind of come into her own by using her brain. She's going to be a hero that will reach for a book or use her intelligence more than a sword."

Watch the promo below for "The Tower" below, which airs Sunday at 8 p.m. EST on ABC.