Quinn killed George and made it to look like a self defense, but the the girl he did it all for, Nadia, left him.
Maria and Tom investigate the Bay Harbour Butcher case and find out that the cabin Doakes died in was rented by one of Dexter's mother's murderers. That, added to the fact that Doakes never had the boat, unlike Dex, makes LaGuerta and Matthews suspicious. So Tom "interrogates" Dexter, although, the conversation feels more like a friendly warning, and so Dexter lies that he saw that Doakes had a boat. Then the Morgans rent the boat storage and plant the evidence all over the place, which convinces Tom, but not Maria. She has a better plan.
Hannah and Dexter grow closer day by day. She teaches him that there's no dark passenger that controls him; he kills her awful father and imagines how they grow old together. But Debra is determined to put Hanna's sweet ass behind the bars and she got a tip: a name of the girl who saw how Hannah poisoned that chancellor Price was talking about.
So there was nothing for Hannah to do than to dissolve a bunch of pills in Debra's bottled water, which made Deb pass out while driving, so she got to a car accident. She didn't die, but to protect her, Dexter gave her the pen Sal Price chewed on before his death with traces of aconite. Hannah got arrested, and she asked her friend to get her something that made her temporarily collapse. When she regained her consciousness, she fled from the hospital she was rushed into.
Maria arrests Dexter for Estrada's murder and interrogates him, but he's very calm. That's all because the evidence she got was planted by Dexter: it was a shirt with Estrada's blood and his wallet, which Dexter took from the evidence room to make LaGuerta look like she tried to frame him. Dex is released.
LaGuerta asks Debra if she was with Dexter at the church on the night of Travis's murder, she said she wasn't, and then Maria shows her the video from the gasoline station. Debra doesn't say anything, but she is scared. Dexter breaks into Maria's apartment and finds warrants for GPS tracking of his and Deb's cell phones. He decides to kill that sneaky bitch to protect his life.
Dexter catches Estrada again, makes him call LaGuerta and kills him afterwards. He wanted to make it look like Maria and Estrada killed each other, but before he finished, Debra comes and ruins everything. Maria asks her to take Dexter down, but Debra shoots her captain instead. And then they go to a party.
So, Debra is a killer now, like Hannah. I wish they could make her conversion look a bit smoother, by making her do a lot more bad stuff along the way. Like that car crash: she could have put those pills into her water herself, to set Hannah up. It feels like there's a huge gap between what Debra was in the beginning of the season and what she is now. Planting evidence and lying to coworkers doesn't lead to killing somebody. It is also quite annoying that the writers never let Dexter murder any innocent people, they always make his women do it.
The story with Hannah is unlikely to be over, I'm sure she and Dexter will try to kill each other next year.
All in all it wasn't the best season, but it was fun to watch. And may I say that Jennifer Carpenter has GORGEOUS hair.