Emma and Hook are dual Dark Ones and Zelena is being her usual evil self.
Camelot: Hook is 5,000 shades of pissed off about Emma stealing his memories, so he steals hers back. Zelena and Hook are best buds now, and he's all gung-ho about being dark.
We see Hook becoming the Dark One, reliving all of his evil events, and coming out of the birthing hole to see guidance Rumple there to mock him. He offers Hook help on the revenge he's been seeking: he wants Gold dead.
Storybrooke: The Charmings and Regina break into Emma's house to see her passed out on the couch. They get her up and find out about Hook's darkness as well as her plans for Zelena. Premeditated murder is frowned upon in Storybrooke, even against the most evil green witch of all time. The Charmings and Regina want their memories back, but Hook stole the dreamcatchers.
Hook visits Gold and Belle, threatening the crap out of Gold. He doesn't just want to kill Gold, he wants to make a game of it - there's a duel at noon on Hook's ship. Dark Hook summons Excalibur, the only thing that can kill him, and tempts Gold with it. He pawn broker visits the Good Gang + Dark Swan to explain his currently deadly situation. Nimue was Merlin's one true love, and also the first dark one, so there's something about her that can probably break through this... right? Emma has been cuffed, and offers Gold protection, but nobody wants to take the magic cuff off. Even Henry says no, clearly distrustful of his mother. Everyone takes off to the library to do some research, but leaves Emma penned up at the house.
Camelot: Charmings, Regina, Lancelot and Merlin come to find the Dark One Spawning Pool of Doom recently used. There are footprints that lead away, but then abruptly stop. Lance's mother is the Lady of the Lake, so it's time to go visit mummy dearest for some answers. David is a little apprehensive about Hook being the Dark One now, with Snow trying to reassure him. Right. Meanwhile, Hook is wandering around the woods with imaginary Rumple messing with his head. Emma finds him and he unloads on her - I escaped the darkness and now you've thrown me back in. Emma wants a future with him, she knows they can get the darkness out of the both of them if they just trust in the ones they love.
Storybrooke: Gold stays behind at the house rather than go to the library. He's finally going to man-up and face this battle with Hook. A lifetime of cowardice and NOW you're choosing to grow a pair? Bad idea. He wants to survive this and earn Belle's love, to give her a more honest life together. If he wins, he'll be at the well where they married... if she meets him there, he'll know she feels the same.
Zelena busts into the hospital like a tornado, looking for her child. Regina is there waiting; Z's daughter is gone and under protection. Regina reminds her that the only reason the child exists is because she's a raging lunatic, and she will stop Z.
Merida is guarding Emma, taking time to point a bow and arrow at her. Hook busts in and slams the ginger to the floor. Hook definitely still has feelings for Emma... anger, disappointment and hatred. He's a free man now, and Emma is no longer the anchor holding him down. She tries to talk him out of it, but he just wants to get what he wants and could care less about the consequences. Hook breaks her heart with some very well placed words; he wants to hurt her like she hurt him.
Camelot: After a nice foresty makeout sesh, Emma and Hook go to find supplies. Imaginary Rumple shows up to toy with Hook's head some more - he asks Emma where Excalibur is. She tries to break the mental connection but she can't. He's angry that she didn't let him die, that she made a decision FOR him. He's never made a decision for her... Emma doesn't believe in Hook and his decision making abilities, and that pisses him off to no end.
Storybrooke: Emma visits Gold in his shop and asks for help: she needs to get the dreamcatchers back. A bit of squid ink remains... use it on Hook to immobilize him and find out where the dreamcatchers are. Gold puts it in his unsafe safe, saying he wants to defeat Hook with honor. Emma thinks she can do this all on her own, and Gold laughs at her confidence. He's right to do so.
Regina and Zelena go to Charming's apartment, finding Robin and baby Hood inside. Regina wants to rip Z's throat out but resists. She explains that she became a better person because of Henry. The love of a child changed her, maybe it can change Zelena. Robin and Regina 'trust' Zelena to be with the child, so long as one of them is present. Zelena holds her girl and hasn't been happier.
Belle, The Charmings and Henry are at the library when Emma secretly poofs herself in. She asks Henry to help her, explaining that she needs help breaking a spell so she can get the dreamcatchers back. He agrees to help her, and Operation Cobra pt. 2 commences.
Camelot: Emma is out looking for Hook and when he doesn't appear, she uses the damn sword to summon him. I mean, as if he's not angry enough as it is, why not freaking FORCE him to you? She tells him that she does believe in him, and she gives him the sword. They have to team up if they are both getting the darkness out of them. She promises to never try and control him again. Then, in the field of pretty flowers where Hook died, the Dual Dark Ones makeout. Again.
Storybrooke: Gold shows up to Hook's ship and the pirate makes it a fair fight, curing Gold's limp. The sword fight begins.
Camelot: Emma and Hook stroll in towards the Good Guys; this is the first time they've seen the two lovebirds as the new Dual Dark Ones. In good spirits overall (for now), they set off to figure out where Merlin is. The wizard is in the back of Granny's, setting up the voicemail via-cauldron that the Good Guys found a couple weeks ago. The dark one that interrupted him wasn't Swan... it was Hook. Evil pirate rips Merlin's heart out and asks for help to get back to Storybrooke. His heart must be crushed in order to get back and cast the curse. Imaginary Gold poofs into Nimue... she lives in all Dark Ones, so she's living in Hook. Ergo, if Hook crushes his heart, it's actually Nimue doing it as well.
Storybrooke: The sword fight carries on, and Gold plunges his sword through Hook. It doesn't do jack shit though, since the sword isn't Excalibur. Nice try, dearie.
Henry used a location spell to find where Hook hid all the dreamcatchers. They're protected by a spell and since Emma has the bracelet of anti-magic on her, she can't do squat. Henry reveals he stole the squid ink from Gold's shop so that they can remove her bracelet. He trusts his mother a bit now, and she promises to make it up to him. I still don't trust her.
Camelot: Emma comes into Granny's to see she's been crossed by Hook. He's taking control of his own fate, and is making sure there isn't a chance in Hell that Emma, or anyone else, will ever be able to control him again. Nimue is helping Hook so that all the Dark Ones can get their happy ending. She tries to talk him out of destroying the heart, as that won't lead him to his happy ending, she will. Hook is basically like "Killian Jones said that crap, and he's loooooong gone now because of you."
Emma, seriously, if you get out of this in one piece, you need to cut yourself off from humanity for a bit.
Hook crushes Merlin's heart and dumps the heart dust into the cauldron. Welp... say goodnight, Merlin.
Storybrooke: Hook is about to finish Gold off, when imaginary Rumple distracts him. Gold knocks Hook down and threatens him with Excalibur, admitting he'd love to kill the pirate... but he'd rather that he live knowing that Gold beat him. Hook poofs away in a puff of red smoke and once again, the decisions of Gold have led to really bad consequences. By no means do I want Hook dead, but damn... he's gone. He's so gone
Gold is waiting at the well and Belle strolls up. She tells him she does love him, but he's broken her heart too many times, and there's so much distrust between them that she can't move on with him. She doesn't know if she wants it to work, and she has some thinking to do.
Camelot: Emma realizes that she can't stop Hook, so that's when she takes his memories. "When you wake up, you'll be the man you were. You'll be the man I love... the man who loves me." You're so wrong. Emma summons everyone that knew Hook was a Dark One into Granny's and steals their memories. She realizes it's all her fault, and knows she must fix it. Emma puts the dreamcatcher to the cauldron full of curse juice, which adds massive memory loss to the mixture. The curse spreads through the land and we see Merida, Aruthur, Gwenivere and Camelot enveloped in it.
Storybrooke: The Good Guys meet in Regina's office to get their memories back, Henry telling them that they can trust her. She restores their memories and they all look at her so differently. Emma also remembers a piece of the puzzle she was missing - she knows what Hook... what they - are up to.
Hook is out by the pond that the Fury tried to drag Robin into. Apparently it's basically a portal to Hell and we all know this is going to end oh so splendidly. Hook takes his hand hook, with a bit of Gold's blood on it, and dips it into the pond. A small, evil-looking boat appears and out steps our dear friend Nimue. She's brought all the Dark Ones. All of them, in the flesh. It's a literal army of Dark Ones.
This episode felt rather sluggish, but I understand that it's setting up for a big finale, and for the big 100th episode extravaganza. We'll see how the Good Guy Gang of Storybrooke gets out of this one.