Ruby/Red Riding Hood (Meghan Ory) teams up with Mulan (Jamie Chung) and Dorothy (Teri Reeves) in the next episode of "Once Upon a Time" season 5.

In a flashback-filled episode titled "Ruby's Slippers," Ruby and Mulan meet Dorothy in Oz, where they will all work together to defeat Zelena (Rebecca Mader). But their plans will be thwarted by their target, which will result in Dorothy's sudden disappearance. Ruby decides to search for her new friend in the Underworld and ask for the help from the Storybrooke heroes to find out where Dorothy is.

The trailer for the episode shows a confrontation between Zelena and her half-sister Regina (Lana Parrilla) regarding Dorothy's whereabouts. "You trapped Dorothy in an unbreakable curse," Regina scolds her sister. But without remorse, Zelena answers, "I'm sorry if I'm good at what I do!"

Meanwhile, the episode will also show that Snow White (Ginnifer Goodwin) and Prince Charming (David Nolan) will do everything that they can in order to be reunited with their son Neal. According to the synopsis for the episode, the couple will come up with a plan to help one of them escape the Underworld to be able to look for their child. Will one of them manage to get out of the Underworld alive to look for Neal in Storybrooke?

The trailer for the episode also shows that Hades (Greg Germann) knows that Snow, Regina, and Emma Swan (Jennifer Morrison) are up to something, and he will catch them red-handed.

"The gods don't like it when mortals play with their toys," the lord of the Underworld tells them.

Guest stars for the episode include Victoria Smurfit as Cruella De Vil, Emma Caulfield as the Blind Witch, Paul Lazenby as Claude, and Gina Stockdale as Auntie Em.

"Once Upon a Time" season 5 episode 18 airs Sunday, April 17, on ABC.