In the Season 4 finale, the police close in on the Trinity Killer, who cautions Dexter to keep away. But Dexter is willing to do whatever he can to get his man, legal or not. Meanwhile, LaGuerta and Batista must answer for their breach of ethics; and Debra becomes privy to something shes not sure she wanted to know.

It is Thanksgiving and Dexter continues to get closer to Trinity. Rita prepares Thanksgiving dinner and Debra continues to investigate the Trinity Killer.

Dexter knows it's only a matter of time before the Trinity Killer's latest handiwork is discovered by Miami Metro. He has to work doubly hard to stay one step ahead of their investigation. Debra is shut out from her own case. She considers risking her career and bending the rules in the name of justice. LaGuerta and Batista realize that by playing by the rules they may have painted themselves into a corner. Dexter gleans relationship advice from a most unlikely source when the friction between Rita and Dexter comes to a head.

Debra recovers from the shooting but is emotionally distraught from Lundy's death and other life events. Dexter realizes that it was the Trinity Killer and not the Vacation murderer as the rest of the police force thinks. He gets closer to the Trinity Killer than he has before.

Dexter finally believes he understands the beast known as Trinity. But when a ten-year old boy goes missing, Dexter is forced to question everything he’s learned up to this point. It’s a race against the clock. And with each step forward, Dexter has to confront the real possibility that he may have more in common with Trinity than he’d like to admit. Secrets abound – from Cody, who is forced to defend one of Dexter’s lies, Masuka, who can’t bring himself to confess what he witnessed at Thanksgiving, and Debra, whose recent discoveries force one of Miami’s finest into making an impossible decision.