ABC’s Once Upon a Time season six episode four picks up the story with Hyde now freed from jail and hunting down Jekyll along with help from the Evil Queen. Titled ‘Strange Case’ and airing on October 16, 2016, the episode begins by showing off Gold’s (Robert Carlyle) new haircut. He looks much more handsome and the Evil Queen (Lana Parrilla) is impressed. He won’t say why he did it and tries to kick the Evil Queen and Hyde (Sam Witwer) out of his shop, but Hyde needs a cameo necklace and isn’t willing to offer anything in exchange. Gold’s unable to kill Hyde and Hyde knows it, so he simply takes the necklace he wants. Hyde says the stories will all be playing themselves out – even if Gold doesn’t like it. Before they leave the Evil Queen agrees not to hurt Belle, but Hyde doesn’t.
David (Josh Dallas) is busy whipping up a ton of pancakes and other breakfast goodies for Emma (Jennifer Morrison), admitting it’s because with Hook moving in he and Snow won’t see her as much. Emma promises the new housing arrangement with Hook won’t change anything. As Snow (Ginnifer Goodwin) and Henry (Jared Gilmore) head off to school, Regina and Jekyll arrive to announce Hyde’s escaped. Hyde left the cameo in Jekyll’s room to taunt him.
Flashback to Dr. Jekyll and the woman who owned the cameo, Mary (Elizabeth Blackmore). Mary introduces her father, Dr. Lydgate (Jonny Coyne), to Dr. Jekyll (Hank Harris) because Jekyll has created a formula that will separate a man’s personality in two – disconnecting the man from the beast. Jekyll needs Dr. Lydgate to help him with membership in a scientific organization in order to obtain backing for his formula. Unfortunately for Jekyll, Dr. Lydgate says no. Mary’s father believes Jekyll’s work is too dangerous. After they leave, Rumpelstiltskin shows up and says he has a way to get Jekyll the backing he needs. Jekyll tells him the serum isn’t quite finished and so Rumple finishes it for him, asking how far Jekyll is willing to go to get Mary to notice him. Jekyll drinks the potion himself and begins screaming.
Hook (Colin O’Donoghue) is all packed up and Belle (Emilie de Ravin) laughs because he only has one chest’s worth of clothing and possessions. He gives Belle a bit of mermaid magic, a shell she can use to call him whenever she needs help or is in trouble. Gold boards the ship and Hook is ready to attack but Belle stops him. He warns them Hyde has escaped and he’s there to protect them. Hook and Belle want Gold to leave, but before he goes he puts a protective spell around the ship so that Belle can’t leave and Hyde can’t get to her.
Snow’s happy to be back teaching school and she meets Shirin (who is actually Jasmine in town to look for Aladdin) by literally bumping into her in the hallway. They exchange pleasantries, with Snow sort of explaining how she got to Storybrooke.
Emma, Jekyll, David, and Regina learn that Belle is now trapped on the ship. They don’t know why Hyde would go after Belle, and David and Emma head out to track down Hyde while Regina goes to ask Gold for help.
And once again we’re back in time with Rumple escorting ‘Hyde,’ who is confused as to why he doesn’t look like Jekyll, to Mary’s family’s party. Rumple explains it’s because he’s just part of a man; the part Jekyll wanted to hide from the world (which is why his name is Hyde).
Hyde introduces himself to Mary and her father as Jekyll’s friend. Mary goes to fetch a drink and Hyde attempts to blackmail Dr. Lydgate for his help.
The Evil Queen and Hyde show up at Dr. Jekyll’s lab, knock out Grumpy who is on bodyguard duty, and check out what it is that Jekyll’s working on.
Regina compliments Gold on his new haircut and can’t believe he resisted the Evil Queen. She tells him Jekyll needs his help with a new serum to destroy Hyde for good.
Back in time, Rumple wakes Jekyll and tells him his other half is very persuasive. Jekyll’s now a member of the scientific academy, just like he wanted. But, there’s still the matter of Mary. If Jekyll swigs more serum, then Hyde can return and sweep Mary off her feet. Jekyll can’t believe his other half has a name, and he asks Rumple what it is he wants. Rumple wants the serum and if Hyde appears again, then Rumple will know for sure it works.
Back at the lab, Regina and Gold arrive to find Jekyll’s work has been destroyed. Jekyll blames Gold for unleashing Hyde in the first place but Gold puts the blame squarely back on Jekyll’s shoulders. Fortunately, Jekyll hid away some of the formula from Hyde and Regina’s excited to see if it works. Gold grabs Jekyll’s heart, saying the serum will be used on Hyde because he needs to protect Belle and his unborn child. Regina is forced to pour the serum on Rumpelstiltskin’s dagger. He tells her she can’t rid herself of her darkness and neither can he. He leaves after tossing Regina Jekyll’s heart.
Hook’s keeping Belle company on board his ship, suggesting they crack open a bottle of rum. She reminds him she’s pregnant, and he looks a bit lost as to further suggestions. Jekyll barges in and tells Hook and Belle that Gold took the serum because he has an arrangement with Hyde and the Evil Queen. Belle sends Hook to help Emma who’s hunting for the Evil Queen, and now it’s just Jekyll and Belle on the ship. He tells her about Mary and how Hyde destroyed their love.
Flashback to a foggy night with Hyde dressed in tails and top hat and Mary wanting to know more about him. He tells her about Jekyll’s feelings for her, but she doesn’t want to be with him. Jekyll’s work dictates he put aside passion and desire and she wants to give into both. She is definitely attracted to Hyde, telling him everyone else was afraid of him at the party but she wasn’t. She wants to know why he’s so free and he doesn’t really know. They kiss, deeply and repeatedly.
Emma and David are searching the forest for Hyde, discussing pancakes and Hook as they look. Emma’s the first to realize they’re not alone and quickly she’s on the attack. She misses when she tries to shoot him with the magically enhanced Taser thing, and Hyde’s able to turn his attention to David as David attacks. Hyde easily tosses David through the air and takes off, only to run smack dab into Gold’s dagger that was covered with serum. It doesn’t work because Hyde knew what Jekyll was planning. Hyde switched vials, tossing the real one to the ground at Gold’s feet. Hyde now has Rumple’s dagger and forces him to take him to Belle.
Gold and Hyde appear at the ship and Gold threatens him, telling him he better not harm Belle. Hyde says there’s a twist: he’s not the monster Gold needs to worry about.
Flashback to Mary’s house and it’s Jekyll who’s in bed next to her when she awakes. She’s confused and frightened, wondering how he got in her room. She wants to know what happened to Hyde, and Jekyll figures out that means Mary brought Hyde into her bedroom – and into her bed. Mary asks if Jekyll hurt Hyde and she figures out he took the serum. She tells him she’ll never love him, calling him weak and mad. They fight and Mary is pushed out the window, falling to her death. Jekyll takes the serum yet again and becomes Hyde. He’s still holding the cameo when he’s found in Mary’s bedroom by her father.
Belle listens to Jekyll’s story and realizes she’s in trouble. But before she can call for Hook’s help using the shell, Jekyll throws it to the floor. She stabs Jekyll and runs but can’t leave the ship. Gold and Hyde are on the dock, but Gold doesn’t understand Jekyll’s on the boat and is attacking her. Hyde, on the other hand, knows exactly what’s going on. Jekyll is going to kill Belle to make the Dark One pay, and Hyde is fine with it because he still wants revenge for Mary’s death.
Fortunately, Hook heard Belle’s call before Jekyll broke the shell. He knocks Jekyll down but Jekyll fights back. Hook gets the better of him, and Jekyll’s speared through the stomach but not dead. On the dock, Hyde begins to command the Dark One but as Jekyll dies, so does Hyde. Hook saved the day! Gold gets his dagger back just as Regina arrives. She believes it was the serum that killed Hyde and Jekyll, but Gold figured out the only foolproof way to kill the doppelgänger is to kill the original. If Regina dies, so does the Evil Queen.
Emma asks Regina if she’s okay and she’s definitely not. She has to die for the Evil Queen to die, and Jekyll still had the capacity for evil even though he supposedly separated himself from it. That means the evil can grow back in Regina at any time. She tells Emma that if she’s not strong enough to keep the evil out, she wants Emma to promise to take care of her.
Gold and Belle have a short heart-to-heart. He wants to make a fresh start and Belle says he keeps finding new ways to create misery for the people around him.
Flashback to shortly after Mary died and Hyde’s trying to convince Rumple he didn’t kill Mary and that Jekyll did. Rumple’s angry that Hyde is so weak and Hyde wants to know why he did this. Rumple won’t tell him.
In Storybrooke, Gold finally reveals the reason behind his actions. He sought out Jekyll to get rid of the feelings he had for Belle. He wanted a way to get them out of his mind. Belle’s upset he locked her on the ship, upset she was almost killed by Hyde, and she tells him she doesn’t need his protection. She also says he cut his hair for himself, because he was tired of looking at the man in the mirror. He almost walks away but then returns to warn her their unborn child will have a mark on his head. If she wants what’s best for their child, she’ll return to him. It won’t be out of love; it will be out of necessity.