The series takes place in the fictional seaside town of Storybrooke, Maine, in which the residents are actually characters from various fairy tales that were transported to the "real world" town and robbed of their real memories by the Evil Queen, Regina (Lana Parrilla), using a powerful curse obtained from Rumplestiltskin (Robert Carlyle). The town's only hope lies with a bail bondswoman, Emma Swan (Jennifer Morrison), the daughter of Snow White (Ginnifer Goodwin) and Prince Charming (Josh Dallas), who was transported from the fairy tale world before she could be cursed. As such, she is the only person who can break the curse and restore the characters' lost memories, aided by her son, Henry (Jared S. Gilmore), with whom she has recently reunited (after giving him up for adoption upon his birth), and his book of fairy tales that holds the key to ending the curse. Henry is also the adopted son of Regina (who is the mayor of Storybrooke), providing a source of conflict between the two women.
Each episode focuses on a character back story. One segment details the character's past life that, when serialized, adds a piece to the puzzle about the primary characters and their connection to the events that preceded the curse and its consequences. The other, set in the present day, follows a similar pattern with a different outcome but also offers similar insights.
The Evil Queen interrupts the wedding of Snow White and Prince Charming to announce that she will cast a curse on everyone and take away all their love so that she is the only one with a happy ending. Many of the fairy tale characters are transported to Storybrooke where most of them have been stripped of their original memories and identities as fairy tale characters.
ABC renewed the series for a second season, which premiered on September 30, 2012. Despite Emma breaking the curse, which brings everyone's memories back, they are not immediately returned to the fairy tale world. With the introduction of magic by Rumplestiltskin/Mr. Gold into Storybrooke, the fates of the two worlds become intertwined, while a portion of the fairy tale world that was relatively spared by the curse is revealed.
The fourth season of the ABC fantasy-drama Once Upon a Time was announced on May 8, 2014. On May 13, 2014, it was revealed that the season would be split into two parts, the first half airing during 2014, and the second half during 2015. It premiered September 28, 2014.
On April 20, 2014, it was confirmed that former Once Upon a Time in Wonderland star Michael Socha would be a series regular as his character Will Scarlet/Knave of Hearts. The final minutes of the third season finale revealed that Elsa will appear in some capacity. Kristoff and Anna from the 2013 film Frozen will also be introduced in Season 4, along with a new main antagonist, portrayed by Elizabeth Mitchell. The second half of the season will see the return of Kristin Bauer van Straten as Maleficent in a recurring capacity for a "major arc".
- Ginnifer Goodwin as Snow White/Mary Margaret Blanchard (Season 1-6)
- Jennifer Morrison as Emma Swan (Season 1-6, regular; Season 7, guest)
- Lana Parrilla as The Evil Queen/Regina Mills (Season 1-present)
- Josh Dallas as Prince Charming/David Nolan (Season 1-6)
- Jared S. Gilmore as Henry Mills (Season 1-6)
- Raphael Sbarge as Jiminy Cricket/Archie Hopper (Season 1, regular; Season 2-present, recurring)
- Jamie Dornan as The Huntsman/Sheriff Graham (Episodes 1-7, regular; Episode 22-present, guest)
- Robert Carlyle as Rumplestiltskin/Mr. Gold (Season 1-present)
- Eion Bailey as Pinocchio/August Booth (Season 1, regular; Season 2-present, guest)
- Emilie de Ravin as Belle/Lacey (Season 1, guest; Season 2-6, regular)
- Colin O'Donoghue as Captain Hook (Season 2-present)
- Meghan Ory as Red Riding Hood/Ruby (Season 1, recurring; Season 2, regular; Season 3 & 5, recurring)
- Michael Raymond-James as Baelfire/Neal Cassidy (Season 2 & 5, recurring; Season 3, regular)
- Michael Socha - Will Scarlet (season 4)
- Rebecca Mader as The Wicked Witch of the West/Zelena (Season 3-4, recurring; Season 5-6, regular)
- Tony Amendola as Geppetto/Marco (Debuts in Season 1)
- David Anders as Dr. Victor Frankenstein/Dr. Whale (Debuts in Season 1)
- Lee Arenberg as Grumpy/Dreamy/Leroy (Debuts in Season 1)
- David-Paul Grove as Doc (Debuts in Season 1)
- Kristin Bauer van Straten as Maleficent (Debuts in Season 1)
- Sarah Bolger as Princess Aurora (Debuts in Season 2)
- Jamie Chung as Mulan (Debuts in Season 1)
- Keegan Connor Tracy as Blue Fairy/Mother Superior (Debuts in Season 1)
- Alan Dale as King George/Albert Spencer (Debuts in Season 1)
- Beverley Elliott as Widow Lucas/Granny (Debuts in Season 1)
- Giancarlo Esposito as Magic Mirror/Genie/Sidney Glass (Debuts in Season 1)
- Anastasia Griffith as Princess Abigail/Kathryn Nolan (Debuts in Season 1)
- Barbara Hershey as Cora/Queen of Hearts (Debuts in Season 1)
- Julian Morris as Prince Phillip (Debuts in Season 2)
- Tim Phillipps as Prince Thomas/Sean Herman (Debuts in Season 1)
- Tony Perez as Henry (Debuts in Season 1)
- Jessy Schram as Cinderella /Ashley Boyd (Debuts in Season 1)
- Sebastian Stan as Mad Hatter/Jefferson (Debuts in Season 1)
- Robbie Kay as Peter Pan (Debuts in Season 3)
- Rose McIver as Tinker Bell (Debuts in Season 3)
- Joanna García as Ariel (Debuts in Season 3)
- Freya Tingley as Wendy Darling (Debuts in Season 2)
- Gil McKinney as Prince Eric (Debuts in Season 3)
- Christopher Gorham as The Wizard of Oz/Walsh (Debuts in Season 3)
- Georgina Haig as Elsa the Snow Queen (Debuts in Season 3)
- Elizabeth Lail as Anna (Debuts in Season 4)
- Scott Michael Foster as Kristoff (Debuts in Season 4)