I think this was one of the better episodes this season. S5 hasn't been my favorite, but I liked a lot of things about this one.
So, Elena wakes up, trapped in her dorm (everyone has gone home for vacation), with no memory of what she did while she was Katherine. She does, however, seem to know about being injected with the ripper virus, but claims that she feels fine. Stefan isn't buying it and tells her she's staying locked up. He's got her on the Damon blood regiment, but she insists she needs more.
Caroline is over at Wes's place trying to find an antidote, when she runs into Enzo. He claims he can get the antidote, but she's got to get Stefan. So, the three of them head over to this fairly remote place where they are surrounded by Travelers. For whatever reason, the Traveler's need to kill Stefan's doppelgänger (I cannot remember why) and they need to link him to them to find him. He's obviously hesitant, as the last time he was linked to someone, he lost his memory. But, for the cure, he agrees to do it. Enzo grabs the antidotes and heads back to hand them over.
By this time, Elena and Damon are talking on the phone, and she's starting to get some symptoms. She's coughing up blood, her nose is bleeding, and she's starting to hallucinate. She's seeing herself do thinks as Katherine that are out of character for her and begging her friends to realize it's her. Then she starts seeing Aaron and is convinced that she killed him when she was Katherine. She's seriously losing her shit. Damon is listening to her freak out and then the line goes dead.
Damon tricks Matt and Jeremy into getting free and races over to Elena. However, over at the dorm, Bonnie and Liv show up and Elena throws a stake through Liv, telling her if she doesn't let the barrier down so she can give her some blood, she'll die. Liv does, and Elena runs off. During her escape, she runs into Luke (who claims the met at the dance, but I don't remember him) and is planning to turn him so she can feed off of him, when she realizes he's been feed from repeatedly. She leaves him and scampers on.
She and Damon meet on the street and she is seriously freaking out about killing Aaron, and Damon is forced to tell her that he killed Aaron. He lost his mind for a bit after she (as Katherine) broke up with him. Just then, Enzo shows up with the antidote and all is right with the virus.
Back with the Travelers, Stefan is starting to be in pain, and Caroline is begging them to stop. They refuse, so she holds a knife up to the girl's throat and threatens her. Once he wakes up, he's fine (even though he jokingly calls Caroline, Rebekah) and Caroline informs him that to get the Travelers to stop hurting him, she agreed to go with Enzo and kill his doppelgänger. Stefan isn't thrilled about this, but she's made up her mind.
At the boarding house, Elena fights with Damon telling him he should have known she was Katherine. He agrees with her, and tells her it's killing him that he didn't know. They both agree that the relationship is toxic, but they love each other. They agree to break up, and one minute later find themselves in bed. I don't know if they are still together or not, but I'm hoping they are. I mean, it is a toxic relationship, but this is TV and it's fun to watch.
Oh, and we find out that Luke and Liv are siblings and apparently Liv already knows magic and is just allowing Bonnie to think she's teaching her. There is way more going on here than they are letting on.