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The next episode of "Once Upon A Time" season 6 will reveal the shocking "untold story" of Mr. Hyde (Sam Witwer) and Dr. Jekyll (Hank Harris).

Episode 4 titled "Strange Case" teases that there is more to it than meets the eye when it comes to the pair as the installment will bring fans back to the time before the serum was successfully concocted.

The "Once Upon A Time" season 6 episode 4 promo shows Hyde revealing to Rumple (Robert Carlyle), who sported the short hair much to the surprise of fans, that he is "not the monster."

As he said these words, Jekyll was seen throwing things at the fireplace. Before that moment, he was seen getting a visit from Gold (Carlyle).

"Looks like someone's lost control of their inner beast," Gold told Jekyll in the "Once Upon A Time" promo, suggesting that he might not be what he seems he is.

"Once Upon A Time" season 6 episode 4 might be looking to change everything viewers thought they knew about Hyde and Jekyll. It will also show how Rumple got involved with the creation of the serum.

According to the synopsis of the episode, "his help comes with a hefty price." It is unclear as to what it is, but it appears to be a big, game-changing one for Hyde.

Back in the present, the Evil Queen (Regina Parilla) and Hyde go about their usual business to get the serum. The former was shown in the trailer stopping by Jekyll's to do some destruction.

Meanwhile, in the midst of the danger and the predicament, Snow (Ginnifer Goodwin) and Emma (Jennifer Morrison) try to live as normal as possible in "Once Upon A Time" season 6 episode 4.

The former prepares to be back to doing what she loves most, which is being a school teacher while Emma is excited to take the next step in her relationship with Hook (Colin O'Donoghue) by having him move in with her.

However, the pirate got himself in a bit of a jam as he attempts to get Belle (Emilie de Ravin) out of the clutches of Gold and into safety.

"Once Upon A Time" season 6 episode 4 will air Sunday, Oct. 16 at 8 p.m. EDT on ABC.