Dexter Morgan is a blood spatter analyst for the Miami Metro Police Department with a secret: he's a serial killer. He lives by a strict code, only targeting other murderers who have escaped justice. His adoptive sister, Debra Morgan, is a driven detective, and his boss, Lt. Maria LaGuerta, is sharp and observant. Dexter's "dark passenger" compels him to kill, a compulsion he struggles to control while maintaining a normal life.

The season's central conflict revolves around the "Ice Truck Killer," a mysterious murderer who leaves victims bloodless. This killer begins to taunt Dexter, leaving gruesome clues that force Dexter to confront his own past and his adoptive father Harry's teachings. The Ice Truck Killer is revealed to be Dexter's biological brother, Brian Moser, who kidnaps Debra. Dexter is forced to kill his own brother to save Debra, ending the Ice Truck Killer's reign of terror. The season concludes with Dexter embracing his dark nature, while Lt. Doakes begins to suspect Dexter's involvement in the killings.

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

Season 1, Episode 1: Dexter - The episode introduces Dexter Morgan, a blood spatter analyst with a dark secret: he's a serial killer who targets other murderers. He begins by killing Mike Donovan, a pastor who murdered young boys. Dexter is intrigued by the "Ice Truck Killer," who leaves victims bloodless, and the episode concludes with Dexter following a refrigerated truck that throws a severed head at his car, hinting at the killer's escalating actions.

Season 1, Episode 2: Crocodile - Dexter stalks Matt Chambers, a drunk driver acquitted of vehicular homicide. While investigating the murder of an undercover cop, Dexter discovers evidence linking Norberto Cervantes to the crime. Debra finds a refrigerated truck with a gruesome message, leading to her promotion. Dexter kills Matt Chambers, disposing of his body. The episode concludes with Dexter finding the Ice Truck Killer has broken into his apartment again, leaving a chilling message.

Season 1, Episode 3: Popping Cherry - While investigating the Ice Truck Killer's latest victim, a dismembered prostitute found in a hockey rink, Dexter is sidetracked by a parolee. Dexter also has flashbacks to his first kill: a nurse who was euthanizing his father, Harry. The episode concludes with Dexter confronting the parolee, Jeremy Downs, in a swamp, leading to Downs's escape after Dexter intervenes.

Season 1, Episode 4: Let's Give the Boy a Hand - The Ice Truck Killer escalates his spree by leaving body parts of his victim, Tony Tucci, at locations significant to Dexter's childhood. This forces Dexter to confront his past and Harry's teachings. Dexter tracks Tucci to an abandoned hospital, but ultimately chooses not to kill the innocent victim, adhering to his code. Meanwhile, Doakes is targeted by Guerrero's men but is saved by a police sting operation.

Season 1, Episode 5: Love American Style - Dexter targets Jorge Castillo, a human trafficker, but discovers Castillo's wife is also involved. He kills both of them after they profess their love for each other. Meanwhile, Debra gets closer to identifying the Ice Truck Killer, and Dexter struggles to understand genuine affection. The episode concludes with Dexter freeing Castillo's victims and unseen by him, someone watches from a car trunk.

Season 1, Episode 6: Return to Sender - Dexter is called to a crime scene only to find his victim, Valerie Castillo, has been returned after he disposed of her body. He suspects the Ice Truck Killer is taunting him, especially when a young witness, Oscar, is found at the scene. Dexter frantically works to cover his tracks by deleting his alias from a list of M99 purchasers, while also trying to deflect his colleagues' suspicions. The episode concludes with Dexter framing Jorge Castillo for Valerie's murder, successfully diverting suspicion from himself, though Oscar's drawing of his "savior" initially appears to be Dexter.

Season 1, Episode 7: Circle of Friends - Dexter is skeptical when police identify Neil Perry as the Ice Truck Killer. Meanwhile, Rita's abusive ex-husband, Paul, returns, and Dexter advises a troubled young killer, Jeremy Downs, who later commits suicide in prison. Dexter realizes Perry is not the real killer, and Debra begins a relationship with Rudy Cooper.

Season 1, Episode 8: Shrink Wrap - Dexter investigates a therapist after three patients die by suicide. He discovers the therapist, Dr. Emmett Meridian, is manipulating his patients. In a pivotal therapy session, Dexter confronts his past trauma, leading to a breakthrough. The episode culminates with the shocking reveal that Rudy Cooper is the Ice Truck Killer, and Dexter finally consummates his relationship with Rita.

Season 1, Episode 9: Father Knows Best - Dexter learns his biological father, Joe Driscoll, has died and left him an inheritance. This revelation causes Dexter to question Harry's honesty about his past. Meanwhile, Doakes faces Internal Affairs for shooting a suspect, and Angel must decide whether to tell the truth about the incident. The episode concludes with Dexter discovering his biological father was murdered, leading him to question his own identity and Harry's teachings.

Season 1, Episode 10: Seeing Red - A blood-soaked hotel room triggers Dexter's repressed childhood memories of his mother's murder. Meanwhile, Rita faces assault charges after hitting Paul, leading Dexter to incapacitate and frame him. The episode culminates with Dexter reliving the traumatic murder of his mother by a chainsaw, revealing the origin of his dark passenger.

Season 1, Episode 11: Truth Be Told - Dexter discovers the Ice Truck Killer is his biological brother, Brian Moser. Rudy (Brian) proposes to Debra, then kidnaps her, intending to kill her. Dexter races to save Debra, confronting Brian in a final, brutal showdown. Dexter is forced to kill his own brother, ending the Ice Truck Killer's reign of terror and his own chance at a true family connection.

Season 1, Episode 12: Born Free - Dexter discovers the Ice Truck Killer is his biological brother, Brian, who kidnapped Debra. Dexter confronts Brian, ultimately killing his own brother to save Debra. The episode concludes with Dexter embracing his dark nature, imagining a world that celebrates his killings, and Doakes beginning to suspect Dexter.