The Ice Truck Killer kidnaps someone close to Dexter, forcing him into a trap that will forever change his life, while Doakes begins to harbor suspicions about Dexter's odd behavior.
Dexter learns more about his dark origins from Dr. Evelyn Vogel, an old family friend, and is reunited with Hannah; Deb suffers the consequences of loving her brother despite everything; Masuka learns he has a daughter.
Dexter comes clean to Deb and finds true love with a fellow serial killer; Deb learns the hard way how difficult it is to keep Dexter's secret; LaGuerta gets closer to pinning the Bay Harbor Butcher killings on Dexter.
Dexter stalks the Doomsday Killer, a vicious, deluded, religious fanatic who displays his victims in grisly, Biblical-themed tableaux; Deb reconsiders her relationship with Dexter; Quinn jeopardizes his partnership with Batista.
Dexter grieves his loss and tries to stay one step ahead of a suspicious Quinn and his own sister, while reluctantly guiding a near-victim (Julia Stiles) of a serial killer who sets out to get lethal revenge.
Dexter has become a husband and father, and finds a potential mentor in a macabre serial killer known as Trinity (Emmy winning guest star John Lithgow) who lives a familiar double life as a suburban family man.
Dexter befriends an assistant District Attorney (Jimmy Smits) who becomes increasingly and disturbingly helpful in Dexter's secret pastime; Debra gets a new partner, Joey Quinn (Desmond Harrington).
Dexter tries to sabotage his department's investigation into his killings and hooks up with an obsessed pyromaniac; traumatized Debra dates the FBI agent assigned to the case; Rita urges Dexter to join a treatment program for addicts.