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.

Dexter awakens after his car accident surrounded by flashing lights. While on a backboard being loaded into an ambulance he remembers killing Benny Gomez and putting the remains in the back of his car.

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 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....

Dexter seems to have finally managed to balance work, family, and his dark passenger. But he knows the balancing act can’t last. A series of murders leads the department to a prominent photographer, prompting Dexter to move him to the top of his list which doesn’t sit well with Harry. But Quinn’s effort to bond with Dexter poses a problem with hunting the new prey. Meanwhile, Debra returns to work and focuses on finding the Trinity Killer, which Dexter fears will put her in danger...

Dexter learns the Trinity Killer may be taking his act on the road when the homicidal maniac plans an out-of-town trip. To gather more insight into the mind of the murderer, Dexter decides to accompany him, but isn’t really ready for some of the shocking revelations of which he becomes privy. Back in Miami, a technicality is keeping Debra off the Trinity case, but she utilizes others in the department to get her opinions registered

Dexter studies those closest to Arthur in an attempt to understand the psychology behind the Trinity Killer’s actions; Rita’s Thanksgiving-dinner plans are complicated when her full house gets even fuller with the addition of some unexpected guests; Batista sees some closure on a troubling old case; and Debra’s continued pursuit of the Trinity Killer may prove a threat to Dexter.

Dexter thinks he knows the ins and outs of the Trinity Killer until a 10 year old boy goes missing.

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.

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 she’s not sure she wanted to know