While Emma (Jennifer Morrison) and Regina (Lana Parrilla) have had their differences in the not-so-distant past, they will have to team up in the upcoming episode of ABC's Once Upon a Time.

Snow Queen Could Be First 'Purely Evil Villain' In Series

Regina decides to join up with Emma in order to seek out the Snow Queen (Elizabeth Mitchell) in the upcoming episode of OUAT, titled Breaking Glass. And, as expected from the title, Sidney Glass (Giancarlo Esposito) is back.

"Regina reluctantly teams up with Emma to search for the Snow Queen after Sidney, who is once again Regina's prisoner in the mirror, discovers her whereabouts and agrees to lead her to the location," a new plot synopsis reads for the episode.

Anna Sets Off On "Quite An Adventure" With Belle; Lots Of "Hiking In Heels"

Meanwhile, Mary Margaret (Ginnifer Goodwin) and David (Josh Dallas) are on a mission of their own.

"With Belle [Emilie de Ravin] babysitting baby Neal, a nervous Mary Margaret and David prepare for their first date night away from their child and find themselves on a mission to track down Will Scarlet, who has escaped from the town jail," the synopsis reads.

Additionally, Elsa (Georgina Haig) and Anna (Elizabeth Lail) finally reunite in the woods of Storybrooke.

As always, the episode will have a flashback to the past, this time with a young Emma befriends a young girl who is a runaway orphan, according to the press release.

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

Watch the promo for the next episode of Once Upon a Time here:

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Once Upon A Time, Television, ABC