Southern Mansion: Rose gives Elena some backstory. There's this group of original vampires called, unoriginally, The Originals. Rose and Trevor have been on the run from them for 500 years, but they can exchange Elena for their freedom, because she's the Petrova doppelganger (remember, Katherine Pierce was originally Katerina Petrova). While the moonstone binds the curse of the Sun and the Moon, sacrificing a Petrova doppelganger will break it. Yikes! I mean first of all, yikes for Elena. I'm suddenly very glad that Katherine's still alive and squirreled away in that tomb. But also yikes, because that would mean all vamps could walk freely in the day and werewolves could and/or would be wolfed-out all the time. No wonder Katherine didn't try to kill Elena. I bet she was saving her as a last ditch substitute if the Originals ever caught up to her. Commercial.
Fortress Forbes: Tyler is waiting inside for Caroline when she gets home. Huh. Some fortress. He is not going to drop the subject. He's a werewolf and... he just *knows* she is, too. It is to laugh -- and so Care Bear does. Tyler is not amused. He throws Caroline across the wall, breaking a mirror in the process and getting pretty damned scary. Caroline vamps out, throws him off her and insists she's not a werewolf.
Southern Mansion: As he's boarding up the windows of the mansion, Trevor calls Elena doppelicious, which kind of makes me like him, so he's going to die, right? He gives us some more exposition. The Originals wanted Katherine. Trevor helped her escape her fate as the first Petrova doppelganger, and so the Originals have been after him and Rose ever since.