Our heroes on ABC's Once Upon a Time will soon discover that their latest endeavor isn't quite what they were anticipating.

Regina & Zelena Dealing With Parent Issues In New 'Once Upon a Time' Flashback?

ABC has released the official synopsis for the midseason premiere of Once Upon a Time, titled "Souls of the Departed." The synopsis reads as follows:

"Emma [Jennifer Morrison], Regina [Lana Parrilla], Mary Margaret [Ginnifer Goodwin], David [Josh Dallas], Robin [Sean Maguire], Henry [Jared Gilmore] and Gold [Robert Carlyle] arrive in the Underworld, on a mission to rescue Hook [Colin O'Donoghue], only to discover that their search will be more difficult than they had anticipated. The inhabitants of the Underworld--all souls with unfinished business and many with quite personal vendettas--hound our heroes at every turn. Meanwhile, in an Enchanted Forest flashback, a familiar face from the Evil Queen's past returns to present Regina with the perfect birthday gift."

'Once Upon a Time' Star Josh Dallas Talks The Upcoming Underworld Arc

This episode also serves as the 100th episode of the series, and as such, we will see many characters who have appeared over its run. Characters like Cora (Barbara Hershey), Sidney Glass (Giancarlo Esposito) and Peter Pan (Robbie Kay) are all returning for this episode. This will also be the first episode with Hades (Greg Germann), the villainous lord of the Underworld, whose look is very similar to that of Hades in the 1997 animated film Hercules.

Once Upon a Time returns for the second half of season 5 on Sunday, March 6 at 8 p.m. ET/PT on ABC.

Watch the new promo for the midseason premiere of Once Upon a Time below:

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Once Upon A Time, Television, ABC