Dexter Morgan, a blood spatter analyst for the Miami Metro Police Department, continues to live his double life. By day, he is a dedicated employee and a devoted, if slightly awkward, husband and father. By night, he is a serial killer who exclusively targets other murderers, guided by a strict code of ethics instilled by his late adoptive father, Harry.
This season delves deeply into themes of faith, belief, and the nature of evil. The central antagonists are Travis Marshall and Professor James Gellar. Gellar is a charismatic and fanatical scholar who interprets biblical prophecies in a terrifyingly literal way. He recruits Travis, a vulnerable and unstable young man, to help him carry out a series of elaborate, tableau-style murders based on the Book of Revelation.
As Dexter investigates these gruesome public killings, he becomes unusually captivated by the killers’ conviction and their grand narrative. He finds himself questioning his own lack of belief and pondering if he is merely another instrument of a higher, darker power. This internal conflict is amplified by the presence of Brother Sam, a reformed killer turned born-again Christian who runs an auto shop. Dexter forms an unexpected bond with Sam, who represents the possibility of redemption and a path away from darkness.
Meanwhile, Dexter’s personal life grows more complex. He struggles to connect with his young son, Harrison, and feels increasing pressure from his wife, Rita’s, now-teenage children. His sister, Debra, has been promoted to Lieutenant and is grappling with the weight of command and her unresolved feelings for her partner, Quinn. Deb also begins secretly seeing a therapist to deal with the severe trauma of nearly being killed by the Trinity Killer.
A major twist occurs midway through the season when it is revealed that Professor Gellar has been dead for years. Travis Marshall has been acting alone, suffering from a severe dissociative identity disorder and hallucinating Gellar as his commanding mentor. This revelation makes Travis a more tragic and complex figure, a puppet master who is also his own puppet.
The season builds toward a catastrophic finale. Dexter ultimately confronts Travis in the planetarium of the Museum of Natural History, where Travis plans to enact the “Final Judgment.” Dexter stops him, fulfilling his own code. However, the season’s most shocking moment belongs to Deb. After finally confessing her love for Dexter to her therapist, she immediately acts on a hunch, finds the kill room, and witnesses Dexter in the act of executing Travis. The season ends with Deb standing in the doorway, her gun drawn, staring in utter horror at the scene before her—her beloved brother, poised with a knife over his latest victim. This single moment shatters her world and sets the stage for a profound upheaval in their relationship.