A new synopsis for the upcoming fourth season of Once Upon a Time has been released, and it looks like Frozen's Queen Elsa (Georgina Haig) could be a villain.

Another Frozen Character Set To Appear In Once Upon a Time

The synopsis for Season 4, posted on Saturday by TVGuide's Natalie Abrams, indicates that the series will delve deep into Elsa's past, in addition to fellow Frozen characters Anna (Elizabeth Lail) and Kristoff (Scott Michael Foster).

"The Storybrooke residents do not know what Elsa has in store for them," the synopsis reads. "Back in the past, in Elsa's kingdom of Arendelle, we will discover what happened to her, her sister Anna and Kristoff, and how and why Elsa became a prisoner of Rumplestiltskin [Robert Carlyle]."

Once Upon a Time: Emma, Snow, Regina & More Battling 'The Ice Queen' In Season 4

Additionally, the synopsis previews the potential return of Regina's (Lana Parrilla) evil side after Emma (Jennifer Morrison) brought Marian (Christie Laing) back to the present in the Season 3 finale.

"Now that Emma has brought back Robin Hood's [Sean Maguire] wife, Marian, and possibly sparked the end of Regina's relationship with him, will she return to her old, evil ways and do everything in her power to get even?" the synopsis asks, hypothetically.

The synopsis also reveals that new fairy tale characters will make their debuts in Season 4 and they will be joined by older characters coming back.

"But not everyone can be trusted," the synopsis concludes, ominously.

Once Upon a Time Season 4 premieres on Sunday, Sept. 28 at 8 p.m. ET/PT on ABC.