Isobel returns. She changes alliances more times than we can even count, but she dies. The short version: Isobel killed herself to make sure she wasn't compelled to hurt Elena anymore.
Damon and Stefan put their house in Elena's name. That way all vampires can't get in without her say-so.
Bonnie visits an ancient witch burial ground so she can harness their power to defeat Klaus. The downside? She will die if she summons all of her power at once, which that will take.
Matt confronts Caroline and makes her tell her everything. But then he doesn't want to know and begs her to compel him to forget, which she does ... only he took vervain so he remembers after all!
Katherine is abducted, waking up in the custody of Klaus' witch, who is performing a spell on Alaric. When he's done, Alaric wakes up, only now he's being possessed by Klaus!
Source: TV Fanatic