Season 4 begins with Dexter a full-fledged family man with a wife, two stepchildren, a new baby and the same hankering to dole out justice to murderers who remain free. Special Agent Frank Lundy seeks his help with a homicidal maniac who's been dubbed the Trinity Killer because he kills in threes.
When a rash of vandalism strikes Dexter's new neighborhood, the increased vigilance of his neighbors impacts his ability to take care of his side business, and it doesn't help matters that he is temporarily unable to drive. As for the Trinity Killer, Dexter is developing an appreciation and admiration for his incredibly impressive technique.
Rita and the children take an out-of-town trip, leaving Dexter behind for some long-overdue rest and relaxation, which he instead channels into chasing down a new victim. But his investigation turns up some surprising results and he begins to empathize with the killer, a policewoman who murdered her entire family.
Dexter decides to take on the Trinity Killer case on his own; Debra takes the blame for an unforeseen event that she had no control over; and Rita discovers one of Dexter's secrets and begins to question just how much she really knows about her mysterious mate.
Dexter steps up his own investigation into the Trinity Killer case when he realizes the police department is making major headway; Debra's frustration over getting shut out of her own case leads her to make compromising decisions that could impact her job. Elsewhere, Batista and LaGuerta have second thoughts about personal and professional decisions that may affect their jobs and relationship; and Dexter gets romantic advice from an unlikely adviser.
When Miami Metro uncovers a string of homicides and connects them to a well-known photographer, Dexter turns his focus on this new perpetrator in hopes of taking a shot at bringing him down. But Quinn's interest in Dexter's nocturnal pursuits puts a crimp in his plan. Meanwhile, the Trinity Killer is still at large and Harry urges Dexter to stay with the case; Debra returns to work and is frustrated by all the undue attention and kid-glove treatment.
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.
Just when Dexter figures he has a full understanding of the heinous Trinity Killer, a 10-year-old boy goes missing and Dexter must rethink his strategy in a race to find the youngster. Elsewhere, Cody feels the need to defend one of his stepfather's lies; Masuka is conflicted about revealing what he witnessed at Thanksgiving dinner; and Debra's recently unearthed secret about one of Miami's finest puts her in a touchy situation.
Dogged Debra's investigative skills have the department on the verge of not only nailing the Trinity Killer, but ultimately exposing Dexter's clandestine and deadly side job. It would also deprive him of the opportunity to deal with the serial murderer in his own way. At home, Rita's decision to confide in her husband doesn't garner the result for which she hoped. And back in the squad room, a breach of ethics has Batista and LaGuerta in hot water.
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.