The sixth season of Dexter picks up about a year after events on the season five finale.
We learn that LaGuerta and Batista are divorced, as she's been promoted via blackmail and is recommending him to take over as Lieutenant.
We also learn that Dexter has purchased the apartment next to him and is using Batista's sister as his nanny. Dex seems more relaxed than ever, killing at will and accepting that this is who he is. We'd led through his high school reunion, for example, which ends in him taking out a former classmate who killed his wife.
We also see Quinn in the bathroom of a bar, preparing to propose to Deb, only for an intruder to enter the establishment and Deb to take him out. Someone catches it on camera phone.
Finally, we meet Professor Gellar and his protege. They arrange a murder in which they slice open a man and leave seven snakes inside of him. The crime scene intrigues Dexter - who also wants to get Harrison in to a private, Catholic school, even though he's a non-believer - and we later see Gellar saying "it's begun."