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.

Dexter finds himself enjoying a brief moment of tranquility. He's finally managed to strike a balance between work, family... and other pursuits. But he's keenly aware that times like these aren't meant to last. When Miami Metro discovers a string of murders connected to a prominent photographer, Dexter sees an opportunity for a clean kill. However, his plans seem to be thwarted every step of the way. Quinn has taken a renewed interest in Dexter's nightlife. Dexter's plans to keep Astor and Cody distracted with new activities soon backfire. Even Harry disapproves of Dexter's choice of pastime, urging him instead to focus his efforts on the Trinity Killer. Meanwhile, Debra returns to work, frustrated to find everyone treating her with kid gloves. She decides to concentrate on her pursuit of the Trinity Killer as well as delving into her father's checkered past - investigations which Dexter fears might lead her to danger - or worse, the truth.

Dexter has always known his father’s Code was meant to protect him from exposure. But after committing a serious mistake, he wonders if it could have been designed to protect him from something even more dangerous – human emotion. When he discovers that Trinity is planning an out-of-town trip, Dexter sees an opportunity to ride along and gain insight into the mind of a fellow monster.

For most people, Thanksgiving is a time for traditions and family. But for Dexter, it’s an opportunity to get closer to his most dangerous adversary yet. As Dexter gains insight into Arthur’s psychology by studying those closest to him, he finds himself drawn into a bizarre and twisted world.

Miami Metro is just one step away from discovering the identity of the Trinity Killer. And Dexter can't let that happen. Dexter must take drastic action to buy himself time to deal with Trinity in his own fashion. Arthur, who still can't understand why Dexter didn't simply turn him in, begins his own bloody investigation into Kyle Butler.

Dexter must make a choice; Quinn stirs up trouble for Dexter but supports Deb.

Dexter tries to focus on the children and fight his dark urges; Debra ends up on Quinn's doorstep.

Dexter and Lumen hunt for their next victim; Debra uncovers new evidence in the Barrel Girl case