Dexter is now a married family man, and Rita has given birth to a baby boy named Harrison. The family has moved to the suburbs, where Dexter struggles to reconcile his new life with his killing urges. Special Agent Frank Lundy returns to hunt down the "Trinity Killer" (John Lithgow) whom he believes always murders three people in a very specific sequence: a young woman in a bathtub, an older mother of two falling to her death, and a father of two bludgeoned to death.
Lundy begins working with Debra and shortly after, an unseen assailant shoots them both, killing Lundy. Afterwards, Debra is barred from working the Trinity case because the shooting is suspected to be the work of Trinity, making Debra a victim. Debra eventually discovers that Trinity was not the shooter, and she is put in charge of the case. Meanwhile, LaGuerta and Batista have renewed their relationship and, in danger of losing their jobs, decide to marry.
While investigating Lundy's data about Trinity, Dexter determines Lundy was killed because he had determined when and where Trinity would claim his third victim. Dexter stakes out the building, and witnesses Trinity bludgeoning a man on a security camera. Dexter follows Trinity to his home, where he is surprised to discover that Trinity is a family man named Arthur Mitchell. Using the alias Kyle Butler, Dexter attends the same church as Arthur and eventually befriends him. While investigating the reasons behind Arthur's crimes, Dexter learns how he manages to reconcile his family life with his killing urges and utilizes that knowledge to deal with his own issues.
Dexter visits Arthur at his home and discovers that he will be traveling to Tampa on another building project, which could be the start of another cycle. Dexter decides to make the trip with Arthur to ensure this doesn't happen. On the way, Arthur takes Dexter to his childhood home and reveals that at the age of ten, he spied on his older sister in the shower out of innocent curiosity. Startled by his presence, she slips in the shower, shattering the glass shower door and cutting open her femoral artery, causing her to bleed to death. His mother committed suicide and his father died soon after. Dexter realizes that Arthur compulsively murders groups of people that bear resemblance to those in his family, recreating their deaths each time.
During Thanksgiving at Arthur's home, Dexter learns that Arthur has been terrorizing his family, locking his daughter away in her bedroom and physically abusing his son. After Arthur attacks his son, Dexter is forced to restrain him. Arthur then goes to see a reporter who has been snooping around the Miami P.D., Christine Hill, who is revealed to be his illegitimate daughter.
Later Arthur kidnaps a boy while being pursued by Dexter. Dexter does some research and finds out that Trinity's pattern begins with the abduction and murder of a ten year old boy first; revealing Trinity's pattern actually cycles in groups of four with the boys always disappearing five days before the first bathtub death. Dexter locates where Arthur is holding the boy and arrives just in time to save him as Arthur escapes.
Debra begins to investigate and eventually arrests Christine after finding that she is Trinity’s daughter. However, Christine is released due to a lack of evidence. Christine secretly contacts Arthur, but he refuses to help her and instead forbids further contact. Arthur also reveals his disdain for her having killed Frank Lundy, which she did to protect her father. Realizing the error of her ways, Christine confesses to Debra. Distraught, Christine seeks forgiveness from Debra, which she refuses. Christine then pulls out a gun and commits suicide in front of her.
One of Harry's old informants takes Deb to the house of Laura Moser. Deb immediately recognizes the house as the home of Brian Moser, the Ice Truck Killer. After piecing together Dexter's connection to Brian and Laura, Deb reveals this information to Dexter, explaining that Laura was his mother and Brian his brother. Dexter feigns surprise, stating that Brian must have tried to get to him through Deb.
Dexter decides that he must draw police attention away from Arthur, so he finds a man whom he can kill and pin the blame on. He then plants evidence and kills the man. Arthur lures Dexter to an arcade where he previously stalked victims and follows Dexter back to the police station, where he learns Dexter's true identity. Soon after, Dexter tails Arthur, but sideswipes a car on the way. Dexter captures and drugs Arthur, but the driver of the damaged car has found Dexter's car, called the police, and in the ensuing confrontation, Dexter is taken in by the police for leaving the scene of an accident and fighting with the driver in the presence of the police. By the time Dexter is released due to cell overcrowding, Arthur is gone from where Dexter had left his unconscious body.
Dexter eventually tracks down Arthur, capturing and bringing him to the bomb shelter where he held the missing boy. Dexter kills Arthur with a hammer and dumps the body. He returns home and picks up a message from Rita. After listening to the message, he decides to call her back. He hears Rita's phone ring from within the house and soon after, he hears Harrison crying. He rushes to the bathroom, where he finds Harrison sitting on the tile floor in a pool of blood while Rita lies dead in the bathtub, ostensibly having been murdered by Arthur.[7]
