The heroes of "Once Upon a Time" are still stuck in the Underworld and they are quickly running out of time. When Hades (Greg Germann) offers Emma (Jennifer Morrison) and Captain Hook (Colin O'Donoghue) a way out from down under in Season 5 episode 20, they jump at the opportunity.

As fans know Zelena (Rebecca Mader) is being held captive by Peter Pan (Robbie Kay) and his son Rumplestiltskin (Robert Carlyle). King of the Underworld has a soft-spot for the lady in green and in his desperation to bring her back, he seeks the help of the heroes in return for removing their names from the tombstones.

Emma and Hook take up Hades' offer and help him out. Unfortunately, Hook is still unable to leave, leading Hades to suggest that the duo take a trip to the depths of Underworld. "It's the only way," he tells the couple in the promo for "Firebird." He further warns the heroic couple that once they are down there, they are on their own.

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Depths of the Underworld surely must mean the worst of worst places, especially if even Hades has no hold over there. The voiceover describes the place as the "land of no escape." One can't help but wonder why these heroes insist on visiting places that are fatally dangerous.

Regardless, it looks like this might turn out to be the last couples' adventure for Hook and Emma, as she is seen tearfully telling Hook, "I don't know how to say goodbye."

To find out what terrible fate awaits Emma and Hook in "Firebird," tune into ABC at 8 p.m. (EST) on Sunday, May 1, or live-stream Season 4 episode 20 via ABC Go! You can also check out the promo for "Firebird" on Television Promos YouTube channel.