Once Upon a Time, ABC
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Concurrent with this season, there was a spin-off series called Once Upon a Time in Wonderland, which ended about a month before the third season of this show did.
The first half of the season takes place mainly in Neverland, while the heroes search for Henry, who is a captive of Peter Pan. (Yes, Pan is a bad guy in this series.) Pan tries to trick Henry into... doing something I don't want to spoil. Meanwhile, our heroes get some help from Tinker Bell (aka "the Green Fairy," who had lost her fairy powers some time ago, for a reason I won't spoil; she's played by Rose McIver). And they're eventually reunited with Neal, who they had thought was dead. And they rescue another of Pan's captives, Wendy Darling (whose brothers were now all grown up and working for Pan in our world, for Wendy's sake). Our heroes also get some help from Ariel (played by Joanna Garcia Swisher, whom I knew from Reba). And eventually they all return to Storybrooke, but Pan is still able to enact another plan... which I won't reveal. But I will say that at the end of the first half of the season, a new curse eliminates Storybrooke, sends everyone back to the Enchanted Forest, and leaves Emma and Henry with false memories. She would believe she had never given him up for adoption, but had raised him herself, and neither of them remembers Storybrooke or any of its inhabitants, or anything that had happened in the first two and a half seasons of the show. The end of the mid-season finale flashes forward one year, with Hook showing up at Emma and Henry's apartment in New York, where he tries to tell her that her family needs her. But of course she doesn't know she has a family, and just thinks he's a crazy stalker or something.
In the mid-season premiere, we see everyone appear in the Enchanted Forest, where they reunite with Prince Phillip and Princess Aurora. They then plan to go to Regina's castle, though Phillip and Aurora stay behind, and they're keeping a secret from the others. The others can't get into the castle, because a spell is protecting it, and Regina is eager to find out who's responsible for that. Meanwhile, they hook up with Robin Hood and his Merry Men. (Definitely not the Disney version, since none of them are animals.) Meanwhile, one year later, Hook continues trying to convince Emma of the truth, and finally gets her to drink a potion to restore her memories. He tells her Storybrooke has been restored and everyone's back, so he and Emma and Henry (who still doesn't know the truth) go there. Emma finds out that, even though everyone there remembers who they are this time, and know it's been a year since they returned to the Enchanted Forest, none of them remember anything that happened in the past year. Nor do they know who's responsible for the curse that sent them back to Storybrooke, or why. (But Snow is now pregnant.) Of course, even before the episode aired, viewers knew the main villain in this story arc would be the Wicked Witch of the West (from The Wizard of Oz; it might be more appropriate to think of her as being from "Oz the Great and Powerful," since that's a Disney movie, but I haven't seen that yet, so I'm not sure. She's played by Rebecca Mader.) In flashbacks throughout the second half of the season, we learn more about her past. But in the present, she's pretending to be a midwife named Zelena (well, I guess Zelena is her name in both realms, just as Regina is always Regina). Anyway, she's ultimately trying to enact a spell to time travel and change her past (which would of course change everyone's past). This isn't something anyone's ever been able to do before, but she's found a way.
I will refrain, as much as I am able, from revealing too much else about the season, but there are some things you need to know. Including the fact that Rumple had died at the end of the first half of the season, but now is alive again, and is being controlled by Zelena, who is in possession of the Dark One's dagger. But some stuff happens that's... weird, and I don't want to even try to explain it, but it ultimately results in Neal dying. And eventually Henry gets his memory back. And Regina starts a romantic relationship with Robin Hood. Eventually the good guys (particularly Regina) defeat Zelena. But... Rumple does something I don't want to spoil, which unwittingly leads to Zelena's time travel spell being reactivated, so Emma and Hook find themselves in the Enchanted Forest, shortly before Snow and Charming first met. (It's all very Back to the Future, and Emma even makes a reference to that.) They spend the final two episodes of the season trying to fix the past after they screw it up, and get Rumple (of the past) to help figure out a way to get them home. Along the way, Emma frees a prisoner of the Evil Queen, a woman who was scheduled to be executed (and had been, in the original timeline). To preserve history, Emma decides to bring her back to Storybrooke and let her family think she died. But that totally messes things up in the present in a very unexpected way (which I will reveal next season). Meanwhile, Charming and Snow (or David and Mary Margaret) now have a new baby, whom they name Neal. And Rumple marries Belle. And at the very end of the finale, we get a glimpse of a character who will be important next season: Elsa, from Frozen. (I hadn't yet seen that when season three ended, but I made sure to watch it before season four started.)