It won't take Once Upon a Time Season 4 very long to introduce a pair of key new characters.

Executive Producer Adam Horowitz Tweeted the front page of the season premiere script yesterday, revealing that the first episode of the fall will be titled “A Tale of Two Sisters.”

This, of course, is a reference to Elsa and Anna, the Frozen siblings who will recur on the ABC hit next year and be portrayed by Georgina Haig and Elizabeth Lail, respectively.

Scott Michael Foster has also been cast as Kristoff, with Once Upon a Time Season 4 digging deep into this animated Disney smash for upcoming storylines.

It's unclear what kind of relationship Elsa and Anna will have when viewers meet them, but casting notes describe the former as someone who can only control her dangerous powers with the calming influence of her sister.

ABC is yet to announce a return date for its fall shows, but you can always watch Once Upon a Time online in order to catch up before official Frozen introductions are made.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.