Dexter is finding it hard to have closure on killing his brother. There is a new FBI agent to head the "Bay Harbor Butcher" case. Debra is finding it tough to put the past behind her.

Dexter realizes it won't be easy to end things completely with Lila. Dexter tries to stay ahead of the investigations as the manhunt for the Bay Harbor Butcher intensifies. An enemy of Dexter figures out his secret.

Dexter must make a massive decision that will have an effect on all those he holds close. Lila meanwhile is back with a secret to blackmail Dexter.

Debra and the homicide unit, Dexter and ADA Miguel Prado are all searching for the same serial killer, who is responsible for the death of Prado's younger brother. Meanwhile at home, Dexter is faced with a decision to make concerning Rita and her kid

As Dexter struggles when Rita pushes to move in together before getting married, Miguel tells Dexter that a husband has murdered his two wives for a financial gain and gotten away with it. Dexter finds this to be the perfect candidate to get away and satisfy that dark urge which every man with relationship pressures go through. Dexter tracks the murderer to Bimini. While he is on the hunt, Rita disappears due to a medical emergency. Meanwhile, Debra works with Anton, her confidential informant to track down one of Freebo’s criminal confidants. But Oddly enough Debra finds herself attracted to Anton.

Dexter undertakes to teach Miguel the Code, but doesn't realize he's whetting his partner's thirst for blood. Meanwhile, Debra is one step closer to finding the Skinner but a secret revealed about Anton sends him packing.

While preparing for his big wedding day, Dexter has to figure out a way to remove Miguel from his life for good. In the meantime, Rita has troubles of her own, when Syl reveals that Miguel has been seeing one of his old flames. Debra finds a new clue that brings her even closer to catching the Skinner.

Six months after the season finale, Dexter is now living with three kids and a wife in a new home. Along with his new son Harrison, the complexities of being a suburban father of three is making it difficult for Dexter to concentrate on his work. His lack of sleep leads him to let a murderer walk by bringing the wrong documents to court and causes him to get into a car crash. Now retired Agent Lundy has returned to finally capture the mythical "Trinity Killer" who has arrived in Miami.

Dexter is hampered by his neighborhood's increased vigilance due to vandalism, his temporary inability to drive himself anywhere, and his admiration for the killing technique of the artful Trinity Killer.