The topic of Dr. Jekyll's (Hank Harris) serum will be present throughout the next episode of ABC's Once Upon a Time.

The next episode of Once Upon a Time, titled "Strange Case", centers on the Evil Queen (Lana Parrilla) helping Hyde (Sam Witwer) in his hunt to nab that special serum. ABC has released a plot synopsis revealing that their quest continues in this episode, while we will also learn more details about how it was created.

"The Evil Queen and Hyde continue on their quest to steal Dr. Jekyll's serum," the synopsis reads. "Snow [Ginnifer Goodwin] looks forward to her first day back as a school teacher; and while Emma [Jennifer Morrison] looks forward to Hook [Colin O'Donoghue] moving in with her, Hook finds himself trying to protect Belle [Emilie de Ravin] from Mr. Gold [Robert Carlyle], who has made sure she can't leave the confines of the pirate ship. Meanwhile, back in the past, Rumplestiltskin helps Dr. Jekyll complete his serum to separate a man's personality into two - good and evil - but his help comes with a hefty price."

Guest stars for this episode include Witwer as Mister Hyde, Harris as Doctor Jekyl, lLee Arenberg as Leroy/Grumpy, Karen David as Shirin, David Avalon as Doc, Olivia Steele Falconer as Violet, Jonny Coyne as Dr. Lydgate, Jordyn Ashley Olson as The Oracle, Elizabeth Blackmore as Mary and Reilly Jacob as Tom Sawyer.

The promo for this episode features Hyde telling Rumple (who is sporting a fresh haircut) that he is "not the monster," implying that the monster is in fact Dr. Jekyll. Rumple tells Dr. Jekyll in the past that the latter has "lost control of his inner beast," which is evident by Hyde's actions in the future.

Once Upon a Time airs Sundays at 8 p.m. ET/PT on ABC.

Watch a promo for the next episode of Once Upon a Time below: