Dexter Morgan, a forensic blood spatter analyst for Miami Metro Police, secretly lives as a serial killer who only targets other murderers. This season, Dexter grapples with questions of faith and his own "Dark Passenger" while raising his son, Harrison. His sister, Debra Morgan, a police officer, is promoted to Lieutenant, bringing her closer to Dexter's hidden world.

The central conflict arises with the emergence of the "Doomsday Killers," Travis Marshall, a religious zealot, and his supposed mentor, Professor Gellar. They commit gruesome, biblically-inspired murders, creating elaborate tableaux across Miami. Dexter investigates, initially believing Gellar is the mastermind. However, a shocking twist reveals that Professor Gellar has been dead for three years, and Travis has been hallucinating him, making Travis the sole, delusional killer. As Travis's acts escalate, he kidnaps Harrison for a final ritual sacrifice.

Dexter confronts Travis, ultimately killing him in his kill room. In a shocking climax, Debra arrives at the scene and witnesses Dexter committing the murder, leaving her stunned and forever changing their relationship.

Season 6 of Dexter covers episodes 1-12. Here's everything that happened:

Season 6, Episode 1: Those Kinds of Things - Dexter attends his 20-year high school reunion, targeting a former classmate who he believes murdered his wife. Meanwhile, Dexter grapples with spiritual questions as he considers a Catholic preschool for Harrison. The episode concludes with Dexter killing Joe Walker, and Debra becoming a hero by stopping a gunman.

Season 6, Episode 2: Once Upon a Time... - Dexter is introduced to Brother Sam, a minister and ex-convict who runs a garage for former inmates. Debra is unexpectedly offered a promotion to Lieutenant and a marriage proposal from Quinn, which she turns down. Dexter investigates Brother Sam, believing him to be a killer, but ultimately targets and kills Julio Benes, a gang member who threatened Sam. Meanwhile, Travis and Professor Gellar prepare their next "Doomsday" victim.

Season 6, Episode 3: Smokey and the Bandit - Dexter targets Walter Kenney, a serial killer from his youth known as "The Tooth Fairy," who collects his victims' teeth. Dexter confronts his own mortality and potential future as an aging killer. Meanwhile, Debra struggles with her new role as Lieutenant, facing challenges from LaGuerta. The episode concludes with Dexter smothering Kenney to death, and Travis and Gellar displaying a disturbing tableau of a murdered jogger tied to four horses, signaling the apocalypse.

Season 6, Episode 4: A Horse of a Different Color - Dexter grapples with faith after Harrison's emergency surgery. Meanwhile, the investigation into the "Doomsday Killer" intensifies with a gruesome tableau of four horses and riders. Travis Marshall, manipulated by Gellar, participates in a deadly trap that results in the accidental death of a waitress, who is impaled by a spiked collar. The episode concludes with Dexter questioning his own beliefs and the nature of faith.

Season 6, Episode 5: The Angel of Death - Dexter investigates the Doomsday Killers, whose latest victim is adorned with angel wings. He suspects Travis Marshall, a museum worker, is involved. Meanwhile, Brother Sam is shot by an unknown assailant. Dexter confronts Travis, who claims Professor Gellar is the mastermind, but Dexter later discovers Gellar has been dead for three years, with Travis hallucinating him. The episode concludes with Dexter sparing Travis, intending to use him to find the truth.

Season 6, Episode 6: Just Let Go - Dexter investigates the shooting of Brother Sam, ultimately discovering that Nick was the shooter. Despite Sam's dying wish for Dexter to forgive Nick, Dexter drowns Nick in the ocean. The episode concludes with Dexter seeing a hallucination of his deceased brother, Brian Moser, applauding his actions.

Season 6, Episode 7: Nebraska - Dexter travels to Nebraska after learning that the Trinity Killer's family has been targeted. He confronts Jonah Mitchell, Trinity's son, who confesses that his sister committed suicide and he then killed their mother in a fit of rage. Dexter, influenced by the ghost of his brother Brian, ultimately spares Jonah, choosing to let him live with his guilt. Dexter then drives away, leaving Brian's dark influence behind.

Season 6, Episode 8: Sin of Omission - Dexter uses Brother Sam's teachings to pursue leads on the Doomsday Killer, while Debra clashes with LaGuerta over a closed case. Dexter confronts Travis, convincing him to help find Gellar by appealing to the "sin of omission." Gellar kidnaps Travis's sister, Lisa, and uses her in a tableau, and Dexter discovers an abandoned church that serves as Gellar's hideout, where he finds Travis chained up. Gellar escapes, but Travis, now fully committed to stopping him, agrees to help Dexter.

Season 6, Episode 9: Get Gellar - Dexter tracks Travis to an abandoned church, only to discover Professor Gellar's frozen corpse in a freezer, revealing Travis has been acting alone and hallucinating his mentor. Dexter realizes Travis is the true killer. Meanwhile, Debra confronts LaGuerta about a cold case, and Louis Greene is revealed to have stolen evidence.

Season 6, Episode 10: Ricochet Rabbit - Dexter discovers Travis is the sole killer, not Gellar, and that Gellar is merely a hallucination. Dexter escapes Travis's trap and frames Gellar for the murders. Travis, with a fanatic couple, kidnaps Holly Benson and cuts her throat, while Dexter confronts and kills "Doomsday Adam" in self-defense. The episode concludes with Dexter realizing the potential for mass casualties from Travis's actions.

Season 6, Episode 11: Talk to the Hand - Dexter thwarts Beth's attempt to release poison gas at Miami Metro, but is exposed himself. Travis escapes after incapacitating Dexter and leaving him on a boat rigged to explode. Meanwhile, Debra grapples with unsettling revelations from her therapist about her feelings for Dexter. Travis escapes Dexter's trap and sets a new tableau, the "Lake of Fire," with Dexter aboard an exploding boat, though Dexter survives.

Season 6, Episode 12: This Is the Way the World Ends - In the season finale, Dexter races against a solar eclipse to stop Travis, who has kidnapped Harrison for a final ritual sacrifice. Dexter confronts Travis in a skyscraper, and after a struggle, Dexter kills Travis. Just as Dexter commits the murder, Debra witnesses the act, leaving her in shock and irrevocably changing their relationship. Dexter is later rescued from the ocean by a passing boat.