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 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.

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.

Dr. Vogel gets abducted by the Brain Surgeon, while Dexter and Debra try and rescue her. Masuka finds out he has a daughter.