The "Once Upon a Time" season 4 premiere is set as filming begins for the highly anticipated "Frozen" arc.

On July 15, "Once Upon a Time" co-creator Adam Horowitz revealed on Twitter that season 4 will premiere on Sunday, September 28, the last in a slew of spoilerific tweets teasing the introduction of the "Frozen" characters.

Watching the monitor on set. #OnceUponATime season 4! Hope to see ya this fall! pic.twitter.com/sXaaAVgrsz

— Adam Horowitz (@AdamHorowitzLA) July 12, 2014

In addition, Horowitz also revealed that the "Once Upon a Time" season 4 premiere will be titled "A Tale of Two Sisters," no doubt signifying the arrival of Anna (Elizabeth Lail) and Elsa (Georgina Haig). "Lost" alum Elizabeth Mitchell will also be playing a secretive, "possibly malevolent" role.

And even before the "Once Upon a Time" season 4 premiere, rumors are already flying about a possible "Frozen" spinoff series, despite the failed attempt of such a venture with "Once Upon a Time in Wonderland."

"We don't have immediate plans for a 'Frozen' spinoff, but they're so inventive, those two…" said ABC president Paul Lee according to E! Online. "Eddy and Adam are endlessly inventive, so they have glints in their eyes of stories that they can spin for years to come. So it certainly doesn't rule out their ability or my ability to go beyond 'Once.'"

"'Frozen' is a wonderful asset. John Lassiter (executive producer of the 'Frozen' film), himself flew up to Vancouver, he's integrally related to the way we're doing that. Adam and Eddie always have a twist on every story, so you can imagine their twist is going to be very different. But no, I think it's the perfect time. We're thrilled with it."