I had the opportunity to work on set of Dexter over the past 2 years. They fired me (I signed a non-compete) but fuck them anyway.
Here is what happens in season 8:
We all have a general idea of what has happened by seeing the trailer. It is semi-accurate but the producers intentionally released that trailer to throw everyone off of what is really going on.
Deb starts using heroin and coke, as well as prescription medications. This starts in episode 1. In the beginning she is pissed off at Dexter and making him feel bad about killing LaGuerta, but eventually she starts helping him kill. In episode 2 Dex and Deb team up to take down Masuka who finds evidence that Deb was the one who killed LaGuerta. Deb does her first ritualistic kill while Dexter watches.
Deb continues to get high, have sex with drug dealers while Dexter gets pissed off because she is too faded/high to stick to the code.
She also quits the force to do her own thing, claiming that it is Dexter that she is upset about but really she ts to get fucked up, and fucked...(hard). She does. We finally get to see Deb's skinny body getting pounded by a guest appearance of Kimbo Slice fucking her 7 ways to Sunday.
There are a lot of side stories like Detective Quinn making Sergeant and turning over a new leaf. Dexter's ex also comes back from the Mid West but does not play a huge part.
Meanwhile Harry's friend and confidant the psychologist enters. Dex doesn't know this and screws her over at the station during some case work and makes her look bad. She's known about Dexter all along but now is in a position to black male Dexter. She begins to make references that she is going to out Dexter (and Deb) so both dex and deb make plans to get rid of her. They needed a plan but knew that a high profile kill like this would draw the attention of a lot of people, so they make it look like a suicide and create a believable alibi.
Dexter's brother has "come back" to see Dexter and is haunting him regularly. He has convinced Dexter to start killing against the code and that it just 'feels so good' so why not do it. Dexter starts by making a few kills of people that just generally piss him off and even slices someone's throat and does not dispose of the body.
Dexter gets the idea to start mimicking the killing of 2 different kinds of serial killers from the past. He starts by killing someone and skinning them to pretend that the skinner might be back. He also mimics kills that were similar to the ice truck killer. All of this is being done in order to throw a crazy footprint wrench into the investigations of Miami homicide.
The FBI's agent Lundy begins to become very concerned over Deb's behavior. He has moved back to Miami in his retirement and wants to get back with Deb. He starts following her and stalking her. He is obsessed with helping her. Deb is now living in o-town with Kimbo Slice, and agent Lundy is not happy about this.
Everyone things that Dexter's step daughter Astor is going to be the little hussy by now, but really it is Cody. Cody is now smoking weed and hanging out with people with gang affiliations in Orlando. Dexter is not happy about this and makes a few moves in an effort to protect him.
Dexter's son is now 5 years old and is now a little man. Dexter's had him tested a few times as he fears that he could be a killer but there are no signs of that whatsoever, Dex is relieved.
In the end the big race is all about Deb vs Dexter. Deb finally gets clean and goes to rehab and they convince her that she needs to be "brutally honest." Dexter knows about this and isn't cool with killing Deb, but makes threats anyway. Dexter pleads with Deb that he will never kill again, but Debora just doesn't beleive him and needs to get honest at the end.
At the end of the final episode, there is a scene where Dexter has been in jail for over 10 years and has been clean and sober of killing, but pans out and shows a dead child molester with a knife through his throat in the yard.
They show Deb taking care of Dexter's son, alongside agent Lundy. I'm totally full of shit and made all this up btw. She's been given full immunity but has never got her job back at Miami Homicide.
