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"Once Upon a Time" returned with its 100th episode on Sunday, March 6. As previewed in many promo videos, Emma and friends visited the Underworld to save Hook. [SPOILER ALERT!] Mary and David found Hook's grave and used the potion, but it only half worked.

In the next episode, their attempt to bring back Hook from the dead continues. But according to a voiceover in a preview for the upcoming outing, to rescue a lost soul, a princess must summon the courage to confront the guardian of the underworld.

Mary finds the help she needs in young Hercules, played by Jonathan Whitesell. Together, they fight against a three-headed beast.

First look at #Hercules & #SnowWhite #OnceUponATime #Once100 https://t.co/wP04vtyBxa pic.twitter.com/Idcmd9yTzW

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— Once Upon a Time (@OncersNet) March 7, 2016

"Labor of Love" airs March 13 at 8//7c on ABC. No official synopsis is released just yet.