As much as I don't really care for the scientist experimentation plot (it's too similar to another show with the same plot I didn't enjoy), I'm 100% here for whatever is going on with Damon. I do like the twist that Damon had tried to shut them down 60 years ago, as he was experimented on back then.
Elena and Caroline are throwing Bonnie a welcome back party. But, she just wants to keep sneaking off to make out of Jeremy. Totally understandable (although, I don't see much chemistry between the two of them). While they are prepping for the party, Caroline makes it known that she doesn't want Damon to come. She's never been pro Elena's relationship with him, but it feels like she's getting more unhappy with it by the episode.
Wes is busy continuing to experiment on Jesse. However, Jesse manages to break free of his restraints, and before Wes can push some emergency button, Jesse attacks him and escapes. He goes back to his dorm and calls Caroline for help. She rushes over and finds him feeding on his roommate. She pulls him off and tries to figure out who made him a vampire. Elena comes over and they work on trying to teach Jesse how to act like a vampire.
Elena sends Damon over to find out what Wes is up to. He decides to tie him to a chair and continue injecting him with terrible viruses so he'll answer Damon's questions. If he answers, Damon will give him some vampire blood to cure him. If not, he lets him suffer. At some point, Elena calls and asks him not to kill him. He reluctantly agrees.
We finally figure out that Wes has been working on making vampires crave the blood of other vampires, so they don't hurt humans and just feed on each other. This is demonstrated when Jesse bites Caroline and freaks out. He rushes over to Wes and demands to know what he did to him. However, Damon with his vampire blood is much more appealing so he attacks him. (He's also somehow much stronger than a normal vampire and is able to overpower him). Elena shows up and Damon begs for help, thinking he's going to die. So, Elena stakes him with a chair leg (or something similar) just as Caroline shows up. She's miserable that poor Jesse is dead. Elena tries to explain he was going to kill Damon, but this is not helping Caroline's anti Damon stance right now.
Later, Damon is cleaning up Wes's stuff and finds some blood labeled in the fridge. He then realizes that he was also experimented on decades ago. He was part of this Augustine vampire stuff. Even though he promised Elena he wouldn't kill him, he decides he has to. But, just then, he manages to escape and hit the emergency button, which rains vervain down into the room. Shortly after, Damon wakes up in some kind of cell that he's obviously been in many years before.
While all this is happening, Katherine is busy getting drunk to try to forget she's dying. Stefan is busy getting drunk trying to forget his trauma. Matt cuts Katherine off, but he's watching the video of him speaking (the language I don't remember- Bulgarian?) and Katherine offers to translate if he keeps serving her. Which he does. He finally finds out about Gregor being inside of him. Katherine grabs Nadia and she makes Gregor tell her that he was just using her to get to Katherine. Then, Katherine kills him (Gregor, not Matt).
Stefan then finds a note that Katherine left for Nadia, which basically tells her she's going to kill herself because she doesn't want to age and die. She jumps off a clock tower, but Stefan is there to catch her. And he basically just tells her to suck it up and deal.
I'm ready to be done with the Travelers stuff, but I'd be fine with keeping Katherine around.