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 sets his sights on a new victim: a murderer so repulsive it seems only fitting that he's a used-car salesman. Dexter's been unable to lie to Rita about his midnight activities, which has her thinking that he's a drug addict. She's threatened to leave him if he doesn't commit to a 12-step program to deal with his supposed addiction. While in this recovery, though, Dexter meets a sexy, mysterious woman named Lila.

Dexter is attacked by the man who murdered his mother. Meanwhile, Lila wants to get closer to Dex, and Debra and Lundy close in on the Bay Harbor Butcher.

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.

Doakes follows Dexter to his next crime scene and shocks him with a revelation about Harry. The Bay Harbor Butcher task force feels they are zeroing in on their man.

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.

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.

Suffering from amnesia following his car crash, Dexter begins searching for Benny Gomez' lost body, with a helping hand from Harry. Meanwhile, Debra struggles with the return of her former lover while Quinn tries to juggle his personal and work life. Meanwhile, Trinity begins stalking his next chosen victim.

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.

Dexter gets some much-needed R&R time with Rita and the kids out of town, leading to his stalking of a new victim until he unexpectedly begins to empathize with his target, a cop that murdered her family.