The final challenge between Dexter and Arthur comes. Debra finds out a long hidden truth. Rita admits that her marriage is troubled. Batista's and LaGuerta's relationship must face consequences.

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.

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

Dexter continues to try to understand The Trinity Killer by talking to his family; he discovers that Authur Miller may not have the perfect mask after all. With Dexter out on the prowl Rita gets a little help with her busy household from her handsome next door neighbor, but his motives may not be all that honorable. Debra continues to investigate the Trinity Killer and may be getting a little too close for her own safety.

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 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 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 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 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 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 misplaces needed evidence in the Trinity Killer case, evidence which could expose his secret. Meanwhile, Miami Metro must juggle two serial killers at the same time, as the Vacation Murders might bring the city's tourism industry to a grinding halt.

Dexter has to fit the newborn baby into his complex life. An old serial killer returns to Miami, and Frank Lundy returns to close the case.