Six months after the season finale, Dexter is now living with three kids and a wife in a new home. Along with his new son Harrison, the complexities of being a suburban father of three is making it difficult for Dexter to concentrate on his work. His lack of sleep leads him to let a murderer walk by bringing the wrong documents to court and causes him to get into a car crash. Now retired Agent Lundy has returned to finally capture the mythical "Trinity Killer" who has arrived in Miami.
Dexter realizes that he has carelessly misplaced crucial evidence that could expose his dark side to the world, while helping to investigate new slayings that Agent Lundy insists are the work of Trinity.
Dexter is hampered by his neighborhood's increased vigilance due to vandalism, his temporary inability to drive himself anywhere, and his admiration for the killing technique of the artful Trinity Killer.
Dexter gets some much-needed R&R time with Rita and the kids out of town, leading to his stalking of a new victim - until he unexpectedly begins to empathize with his target, a cop that murdered her family.
Dexter is inspired to investigate the Trinity Killer on his own, while Debra blames herself for an event that was out of her control, and Rita realizes how little she knows her husband after discovering one of his secrets.
Dexter knows it's only a matter of time before Miami Metro discovers the Trinity Killer's latest handiwork. Therefore, he has to work doubly hard to stay one step ahead of their investigation. Meanwhile, Debra becomes frustrated when she finds herself shut out from her own case. She considers bending the rules in the name of justice, risking her career in the process. LaGuerta and Batista come to regret a major decision, and realize that by playing by the rules they may have painted themselves into a corner. And when the friction between Rita and Dexter comes to a head, Dexter gleans relationship advice from a most unlikely source.
Dexter finds himself enjoying a brief moment of tranquility. He's finally managed to strike a balance between work, family... and other pursuits. But he's keenly aware that times like these aren't meant to last. When Miami Metro discovers a string of murders connected to a prominent photographer, Dexter sees an opportunity for a clean kill. However, his plans seem to be thwarted every step of the way. Quinn has taken a renewed interest in Dexter's nightlife. Dexter's plans to keep Astor and Cody distracted with new activities soon backfire. Even Harry disapproves of Dexter's choice of pastime, urging him instead to focus his efforts on the Trinity Killer. Meanwhile, Debra returns to work, frustrated to find everyone treating her with kid gloves. She decides to concentrate on her pursuit of the Trinity Killer as well as delving into her father's checkered past - investigations which Dexter fears might lead her to danger - or worse, the truth.
Dexter has always known his father's Code was meant to protect him from exposure. But after committing a serious mistake, he wonders if it could have been designed to protect him from something even more dangerous - human emotion. When he discovers that Trinity is planning an out-of-town trip, Dexter sees an opportunity to ride along and gain insight into the mind of a fellow monster. Even so, Dexter isn't prepared for the startling revelations Trinity makes along the way, revelations about his own past which only serve to convince Dexter of what he must do. Meanwhile, back at Miami Metro, Debra refuses to let a technicality keep her off the Trinity case. She works through others to ensure her voice is heard - that is, until she realizes that her single-minded pursuit of the Trinity Killer may have blinded her to the truth.
Dexter uses the Thanksgiving holiday as an opportunity to get closer to his most dangerous adversary up to now. Upon getting to know those closest to Arthur, Dexter finds himself drawn into his bizarre and twisted world. In the meantime, Rita prepares Thanksgiving dinner for what turns out to be a few extra guests. Batista finally closes as old case, while Debra continues investigating the Trinity Killer throughout the holiday.... which puts Dexter's safety at risk, and exposes unexpected revelations about another person close to her.
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.
Dexter's worst fears have been realized. Because of Debra's unrelenting pursuit, Miami Metro is just one step away from discovering the identity of the Trinity Killer. And Dexter can't let that happen. Arthur's arrest would not only deprive Dexter of a satisfying kill, it would also expose the secret life Dexter's been leading in his pursuit of this monster. Dexter must take drastic action to buy himself time to deal with Trinity in his own fashion. Meanwhile, Rita decides to confide in Dexter, which doesn't go as well as she'd hoped. LaGuerta and Batista's breach of ethics paints them into a very tight corner. And Arthur, who still can't understand why Dexter didn't simply turn him in, begins his own bloody investigation into Kyle Butler.
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 she’s not sure she wanted to know.
Songs: Frankie Avalon -- Venus