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 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.