The season 4 finale of ABC's Once Upon a Time will be tumultuous as the battle between good and evil reaches its climax.

Will Regina Find Her 'Happy Ending' In The 'Once Upon a Time' Finale?

Gold (Robert Carlyle) and the Author (Patrick Fischler) have something up their sleeve in the season finale of Once Upon a Time. ABC has released a plot synopsis for the final episode of the season, which is titled Operation Mongoose.

The finale will feature the big battle as well as a shocking twist.

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"The Author proves to be a formidable wild card and forges an alliance with Gold. Emma [Jennifer Morrison], her parents, Hook [Colin O'Donoghue] and Regina [Lana Parrilla] scramble to stop them, but when Gold and the Author turn the tables on heroes and villains alike, the prospect of any happy outcome appears worlds away," the synopsis reads.

Henry (Jared Gilmore) will also have a big episode as he attempts to "save his family before the story's final page is turned. It's a race to the finish, and everything culminates with a shocking twist that will leave the residents of Storybrooke reeling, in the captivating two-part season finale of Once Upon a Time," the synopsis concludes.

As the promo for the finale suggests, it looks like everything will be turned on its head, with Gold acting like a hero, Snow (Ginnifer Goodwin) acting like a villain. It remains to be seen if they will stay that way beyond the finale.

Once Upon a Time airs Sundays at 8 p.m. ET/PT on ABC.

Watch the promo for the season finale of Once Upon a Time here:

Tags: Once Upon A Time television ABC