Dexter Morgan, a Miami Metro Police Department blood spatter analyst, is living a double life. After his day job with the police department, Dexter moonlights as a serial killer, hunting and killing criminals who slip through the justice system. Dexter's sister, Debra, a vice squad officer, pulls Dexter into her world when another serial killer is killing prostitutes and leaving their bloodless bodies dismembered in various locations around Miami. Meanwhile, Dexter hunts a man who made snuff videos and killed a mother of two.
As Dexter stalks his next victim, a drunk driver who is about to be acquitted for vehicular homicide that resulted in the death of a teenage boy, the Ice Truck Killer strikes again and later gets in touch with Dexter. Meanwhile, when a cop is found murdered, Dexter helps Doakes and Debra investigate a crime boss who they believe is responsible.
"The Ice Truck Killer" strikes again and new homicide officer Debra is assigned the case, but she gets reprimanded for violating the chain of command; flashbacks offer insight into Dexter's homicidal tendencies and take him back to his first time "taking out the garbage"; Doakes gets on the bad side of a drug kingpin.
The Ice Truck Killer leaves severed body parts from his victims at locations that are linked to memories from Dexter's troubled childhood, which begins to get into Dexter's head.
The homicide division is handed an unbelievable lead when the Ice Truck Killer's latest victim is found alive but mutilated; Dexter is on the trail of a person who traffics in humans.
The Ice Truck Killer leaves a clue at the scene where Dexter has dispatched his latest victim, and Dexter is worried that someone might have witnessed his killing. Rita is trying to deal with her ex-husband.
The squad identifies the Ice Truck Killer; Rita's ex-husband, an abusive addict, is paroled.
Dexter appears to be connected to a psychiatrist suspected of murdering a suicidal patient.
While on a weekend getaway with Rita, his sister and her new beau, Dexter learns that his biological father, thought to be dead for 30 years, has just passed away, and that he's willed his son everything he owned.
The Ice Truck Killer prepares a crime scene for Dexter: a hotel room all covered in his previous victim's blood. But Dexter does not appreciate this gift, because his childhood memories hunt him.
The Ice Truck Killer sets a trap for Dexter by kidnapping someone close to him; Dexter's increasingly odd behavior piques Doakes's interest.
Dexter figures out who the Ice Truck Killer is, along with a dark secret from his troubled past. Meanwhile, La Guerta is replaced with a new boss and Angel figures out a way how to track the killer while he's still in hospital.
Dexter finds himself not "on his game" after the death of his brother, the ice truck killer. He fails and shows mercy and then fails to prepare his victim after that. In the meantime his sister returns to work even though she still has some signs of mental trauma from her brush with death. A dramatic find at sea by treasure hunters triggers different feelings from himself and his sister.
The discovery of the dumping ground off the coast brings in a renowned FBI agent for solving practically impossible cases. Dexter continues to struggle but finally comes to terms with the death of his brother. Dexter reveals to Rita the truth about Paul's return to the prison system.
Dexter's sudden inability to deceive has him pursuing a slick liar as his next victim, while Rita believes he's concealing a drug problem and forces him into a 12-step program where he meets Lila, a seductive new woman.
While pretending to be a drug addict, Dexter gets an N.A. sponsor to please Rita. But when the sponsor turns out to be a beautiful woman who may have more in common with Dexter, Rita is not happy. Things get worse for Rita when her estranged mother comes for a visit. Meanwhile, Dexter must find a way to lead the all-too-smart FBI Agent Lundy, now working with Miami Metro PD, off his trail.
Dexter learns that one of the men who killed his mother is still alive, and decides to confront him with Lila so that his addiction does not get the best of him.
A copy-cat killer inspired by the Bay Harbor Butcher begins to stalk Miami criminals, much to Dexter's chagrin. As Dexter tries to stop the fake, he tries to recover a video that might expose him for who he really is.
The Bay Harbor Butcher sends the Special Task Force a written letter, making them all on edge. Continuing with his plan, Dexter tries to be more "pro-active" in keeping Lundy away from him, but things become complicated when Lila begins to take a toll on Dexter's personal life.
After there is a fire at Lila's house, Dexter finds that she has become more attached to him than he's comfortable with. Meanwhile, when "recovery" doesn't work for the man who killed his mother, Dexter pays the man a visit. At the station, Deb and Lundy find a clue that makes them suspicious that the Bay Harbor Butcher might be closer to them than they think.
As Dexter tries to break away from Lila, he finds that she's not ready to let him loose. Meanwhile, Lundy's search for the Bay Harbor Butcher continues, making Dexter nervous. However, Dexter's worst enemy discovers his secret.
Dexter gains the upper hand when Doakes follows him to his latest kill. But when Doakes reveals a secret about Harry, Dexter's world falls apart. Meanwhile, Rita begins to let Dexter back into her life and the Miami PD narrows in on the Bay Harbor Butcher.
Dexter begins to dodge bullets, both figurative and literally. Now, Dexter must decide his own fate, as well as those he cares about most. Lila tries to blackmail Dexter into becoming her lover, but makes a discovery that changes their lives forever.
The Bay Harbor Butcher case is closed, but Dexter continues to deal with his issues with Lila and Doakes.
Dexter continues to live his life according to his newly revised code. Deb is asked to help IA by investigating another officer. Batista is promoted to Sergeant ( Doake's old title). While trying to apprehend a new victim, Dex stumbles upon someone not in the plan. After an altercation, Dex is forced to kill the unknown man. Immediately after, the unknown man is recognized as the ADA's brother. The ADA, Miguel Prado (Jimmy Smits), invites Dex to his slain brother's wake. Dex blows off Deb for their annual celebration of Harry's birthday. Rita tells Dex she is pregnant.
Debra and the homicide unit, Dexter and ADA Miguel Prado are all searching for the same serial killer, who is responsible for the death of Prado's younger brother. Meanwhile at home, Dexter is faced with a decision to make concerning Rita and her kids.
Dexter and a pedophile have a run-in; Dexter is unable to restore Freebo's good name after he is accused of a murder he didn't commit. Also, Dexter's friendship with Miguel weighs on his mind.
Ramon's career may be in jeopardy due to his obsession with Oscar's killer. Meanwhile, Dexter does a great job in keeping Ramon off balance by manipulation. Deb refuses to "be a rat" after Internal Affairs presses her to come up with more dirt on Quinn. Also, Rita and Dexter decide it's time to tell the children about her "condition," and Angel's quest for companionship nearly get's him in deep water.
As Dexter struggles when Rita pushes to move in together before getting married, Miguel tells Dexter that a husband has murdered his two wives for a financial gain and gotten away with it. Dexter finds this to be the perfect candidate to get away and satisfy that dark urge which every man with relationship pressures go through. Dexter tracks the murderer to Bimini. While he is on the hunt, Rita disappears due to a medical emergency. Meanwhile, Debra works with Anton, her confidential informant to track down one of Freebo’s criminal confidants. But Oddly enough Debra finds herself attracted to Anton.
Dexter tests Miguel to see if he is on his dark-side job. Rita questions her career path after being fired from her job. Debra believes she in some way caused another homicide while working on her current case.
Dexter angers Miguel when he says no to his latest offer, who's unsure the next target lives up (or down) to Harry's Code. Meanwhile, an elderly, deathly ill Camilla asks Dexter to kill her, insisting she would never ask a friend to do something against his conscience. Also, Rita discovers that Sylvia believes her husband Miguel has had an affair; and Anton is told he is bait to get at Skinner.
Miguel brings Dexter another “project,” but this time he wants to get his hands dirty. Dexter realizes it’s time to teach Miguel the Code – for his own safety as well as Miguel’s. As Dexter walks Miguel through the process, Dexter both struggles with, and enjoys, letting Miguel into his private world so completely. But Dexter doesn’t realize that this has only whetted Miguel’s appetite. Rita finds herself losing control because of the pregnancy. She becomes worried that Dexter may get fed up with her. Because of her hard work, Debra finds herself one step closer to catching the Skinner. But after she finds out a secret about her new boyfriend, Anton, he disappears on her.
Miguel and Dexter's friendship is impacted when Rita confronts Miguel about his alleged infidelity. Believing Skinner has abducted Anton, Debra pulls out all the stops to find him.
Dexter feels like he’s making a deal with the devil when he feigns civility around Miguel after his duplicitous ways. Dexter finds himself in a series of chess-like moves against his former friend, all while hoping that Miguel doesn’t get the upper hand on him. Meanwhile, Dexter watches helplessly as Miguel draws Rita closer through a wonderful wedding gift. Debra, now back with Anton after his close call with the Skinner, wonders if her relationship with Anton is worth putting her career in jeopardy. After all, is it smart for everyone to know that she is dating a former confidential informant?
While preparing for his big wedding day, Dexter has to figure out a way to remove Miguel from his life for good. But how does Dexter manage to do this when Miguel is not only a highly placed D.A., but also his Best Man? Rita has troubles of her own, when Syl reveals that Miguel has been seeing one of his old flames. Meanwhile, Debra finds a new clue that brings her even closer to catching the Skinner.
Dexter’s focus shifts to calming his emotionally unstable brother, Ramon, now that Miguel out of the picture. Now that Ramon has calmed down, Dexter feels he can breathe once again. All that is left now is his upcoming marriage to Rita, until another deadly surprise shows it's ugly head. Meanwhile, Batista informs Debra that she is going to received her Detective's shield, but is instantly blindsided when she receives information which may end her promotion.
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.
Dexter faces the repercussions of Rita's murder and contemplates running from all of it. Astor and Cody return from Disney World to the news of their mother's death. Deb finds herself handling all of Dexter's responsibilities and an unexpected turn in her relationship with Quinn, who has theories of his own regarding the investigation.
Dexter tries to focus on the children and fight his dark urges; Debra ends up on Quinn's doorstep.
Dexter hires a nanny; Debra is annoyed with her new partner; Quinn continues to investigate Rita's murder.
Dexter finds himself in a strange situation when, instead of taking a life, he has to save one. Deb has a scary confrontation with a key suspect in the Santa Muerte murder cases. Quinn continues following up on the strange similarities between 'Kyle Butler' and Dexter Morgan.
When Dexter is saddled with an unwanted conspirator, he is forced to make some hard decisions about her fate; meanwhile, Dexter wonders if Rita's death has awakened a darkness inside Harrison; due to Quinn's recent suspension, Debra works the Santa Muerte case alone, leaving Quinn time to enlist an old friend to look into Dexter.
Trying to get his life back on track, Dexter hunts a new target, but is distracted and drawn into a precarious situation while helping his troubled acquaintance; Batista discovers an interesting lead in the Santa Muerte murder cases; Quinn learns some new information about Dexter from a questionable source.
While helping Lumen, Dexter finds himself on a collision course with Debra and the rest of Homicide when he is called to investigate a horrifying crime scene; the Santa Muerte case leads to a violent standoff between Debra and the killers.
While attending a self-help seminar, Dexter and Lumen vet and stalk a violent killer; Debra gets into some unexpected trouble from the fallout of the Santa Muerte case, forcing Batista to make a difficult decision.
While in the midst of hunting down their next victim, Dexter and Lumen are surprised by the return of an unannounced guest; Debra is assigned to the file room, but still manages to uncover new evidence in the Barrel Girl case.
Dexter and Lumen go after the final rapist while they're also closing in on Jordan Chase, but Jordan is closing in on them as well. Meanwhile, Miami Metro searches Cole Harmon's house and discovers some disturbing DVDs.
Dexter and Lumen's plans are momentarily thwarted when Dexter's forced to do damage control after he concedes that someone, outside of Miami Metro, has taken an interest in them. Debra's speculation on the missing Barrel Girl suspects begins to take shape. Quinn finds himself more involved in a homicide than he'd like to be.
In the season finale, Dexter's situation grows desperate when he discovers that Lumen's been set up. Despite knowing he's being baited into a trap, Dexter risks everything to make sure he doesn't fall into the same mistake again. In the Barrel Girls case, Debra lets her personal feelings lead her instincts once she concludes that vigilantes are more than just a theory. Quinn finds himself in a troublesome situation, which only Dexter can help him out of.
In the sixth-season premiere, Dexter shows up at his 20th high-school reunion with the intention of confronting the former prom king. Elsewhere, an investigation into a heinous murder with religious overtones leads Dexter to ponder spiritual matters and wonder about his son's legacy. And an unexpected situation results in Debra becoming a hero.
To solve a mysterious and macabre murder, Homicide brings in Brother Sam, a minister with a criminal past whom Dexter recognizes as a fellow killer that might have found a cover even better than his own; now a local hero, Debra is surprised by two life-changing proposals.
Dexter is confronted with a sobering glimpse of his own potential future when a serial killer from his past makes a startling reappearance; Debra is uncomfortable in her new job; Travis struggles to keep his mentor happy, as he and Gellar prepare a new twisted tableau, ensuring Debra's next task will be a daunting one.
Much to his own surprise, an emergency with Harrison and a new tableau from Gellar and Travis has Dexter leaning on Brother Sam and an unexpected winged messenger for support as he questions the idea of faith; with proof of a religiously motivated killer, Homicide hunts for a zealot, with Debra giving her first official press conference.
With the help of his newfound friend Brother Sam, Dexter wonders if there is light within him to counter the darkness, while the search for the Doomsday Killers takes him in a new direction; Batista and Quinn pay a visit to the university where Professor Gellar taught; due to departmental regulations following the shooting, Debra is forced to begin therapy.
Dexter is caught up in a very personal case that awakens the needs of his Dark Passenger; Debra feels overwhelmed by her new Lieutenant duties, made all the more complicated when she finds out Quinn slept with a witness in the Doomsday case.
Dexter finds himself teamed up with a fellow Dark Passenger when he takes a road trip to Nebraska to tie up some loose ends from the past; Debra deals with the complications of her promotion as she leads the Doomsday investigation.
Dexter uses lessons he learned from Brother Sam to follow up on some new leads in the Doomsday investigation; Debra butts heads with Captain LaGuerta over the case of a dead call girl.
Dexter receives help from an unexpected source while hunting the Doomsday Killers and staying one step ahead of Homicide; Debra discovers something new about herself in therapy.
Dexter tries to figure out the Doomsday Killers' next victim before it's too late; Debra has a strong reaction to a crime scene, which leads her to the realization that she leans too heavily on her brother.
Dexter finds that in order to catch the Doomsday Killers, he must create a macabre tableau of his own; Debra's battle with LaGuerta over the case of the dead call girl boils over, and her therapist makes an unnerving suggestion.
Dexter and Homicide race against a lunar eclipse to catch the Doomsday Killers before their final gruesome act; Debra struggles with a new emotional reality.