Dr. Jekyll (Hank Harris) and Mr. Hyde's (Sam Witwer) untold story will be revealed in the upcoming episode of "Once Upon a Time." What kind of dark secrets are they hiding behind their peculiar transformation?

The promo for the episode titled "Strange Case" is out and it is an episode dedicated to the good doctor and his alternate evil personality. According to the teaser, the viewers will learn the full story of the day Dr. Jekyll unleashed his inner beast. The Evil Queen (Lana Parrilla) and Rumple/Gold (Robert Carlyle) are shown, suggesting that they know a lot about what truly happened. The fact that the troublemakers of Storybrooke were there when the doctor's experiment went wrong is not surprising. There is an interesting part though when Mr. Hyde tells Rumple about one little twist. He said he is not the monster. What could he mean? Is Rumple hiding another terrible secret?

Last season, Rumple made a pact with the villain to save Belle (Emilie de Ravin). His ladylove would not wake up due to the sleeping curse. Mr. Hyde told him that he knew of a way to lift the spell, but it would come at a price. Dr. Jekyll's evil alter ego wanted Storybrooke and Rumple obliged in order to free Belle.

Executive producer Edward Kitsis previously said that the episode on Mr. Hyde would not span the first 11 episodes of the season since the show would be returning to the old format of storytelling.

The EP shared with Entertainment Weekly: "This season is going to be different from past because it's not going to be Hyde comes to town, we fight with him for 10 episodes, and then in the winter finale, he dies and we move on. We are changing around what we're doing this year and going back to that season 1 mentality of small town stories and smaller arcs." 

"Once Upon a Time" season 6 airs on Sundays, 8 p.m. EST on ABC.