Emma (Jennifer Morrison) and Hook's (Colin O'Donoghue) relationship will be put to the test in the next episode of ABC's Once Upon a Time.

Emma and Hook will plumb to the depths of the Underworld in the next episode of Once Upon a Time, titled "Firebird". ABC has released a synopsis for the new episode detailing what's ahead for Emma and the rest of our heroes:

"Hades [Greg Germann] turns to the heroes to ask for help with getting Zelena [Rebecca Mader] back from Rumplestiltskin [Robert Carlyle] and Peter Pan [Robbie Kay]. In return, he offers to take all their names off their tombstones. However, when Hook is still unable to leave, he and Emma must journey into the depths of the Underworld. Meanwhile, Cruella De Vil [Victoria Smurfit] is determined to keep the heroes trapped in the Underworld. In flashbacks, Emma searches for answers about her family and makes an unexpected friendship."

Guest cast for this episode is Emma Caulfield as the Blind Witch, Rya Kihlstedt as Cleo, Geoff Gustafson as Stealthy, Bronwen Smith as Cashier, Max Chadburn as Tasha, in addition to Germann, Kay and Smurfit.

The promo for the episode features Hades telling Emma and Hook, "it's the only way, and once you're down there, you're on your own," referring to the Underworld. The lovebirds, who are desperate to leave the Undeworld, are clearly in danger by the end of the promo. This leads to Emma tearfully telling Hook, "I don't know how to say goodbye."

This episode was written by Jane Espenson and directed by Ron Underwood, and is set to air Sunday in the regular Once Upon a Time timeslot.

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:

Tags: Once Upon A Time television ABC