Well, IIRC, it starts with Deborah's FBI agent lover (Lundy) coming back to Miami to investigate a serial killer whom he suspects has been active for years. He calls the killer "Trinity Killer" because he kills in cycles of three - a young woman in a bath tub with her femoral artery sliced open, a middle-aged mother of two jumping to her death, and an older father of two bludgeoned to death.
Dexter finds out that a recent bathtub death occurred in the same house where a similar murder happened 30 years earlier. He's intrigued by the serial killer's ability to evade law enforcement for so long, and thinks he can learn something from him on how to survive.
Then Deborah and Lundy are shot. Lundy dies, but Deborah survives. However, she didn't see the shooter, who shot her from behind. She thinks that the "Trinity Killer" was the shooter because Lundy was getting close to solving the case.
Dexter, working on another lead using Lundy's material, manages to find out where the "Trinity Killer" will strike again. He goes there, but is too late to stop the killer from bludgeoning the victim to death. He follows the killer to his house, but when he arrives, he finds out that the killer has a wife and 2 teen-aged kids. He feels that he can learn from Trinity on how to balance having a family with being a serial killer, something Harry's code tells him will not work, and he works to get close to him.
Dexter uses an alias to approach Trinity for advice, and after some initial good advice from Trinity that he takes, Dexter begins to realise that Trinity's not really a happy family man. He's actually an abusive psychopath whose family is deathly afraid of him. Dexter is determined to kill Trinity with his own hands, but a series of events causes him to fail to get the opportunity.
Trinity then kidnaps a 10-year old boy, whom Dexter manages to save. He finds out that Trinity actually starts his killing cycle by burying a 10-year old boy alive first to "preserve the child's innocence". Meanwhile, Deborah realises that her shooter wasn't a tall man that fits Trinity's profile, and suspects that the shooter is actually the reporter lover of her partner. The reporter's DNA shows that she's the daughter of Trinity, and they hold her for questioning.
Dexter is still determined to be the one who kills Trinity, so he plants evidence to mislead the Miami PD into chasing the wrong suspect. The reporter is released, but she later admits her guilt to Deborah. Deborah refuses to forgive her, and the reporter commits suicide in front of her.
Trinity manages to find out Dexter works in the Miami PD and confronts him in the police station, where he finds out his real identity. Dexter chases him and manages to sedate him in his van, but before he can kill him, he is stopped by the sheriff department because of a traffic offense. He becomes violent when they are determined to pursue to take action against him. He's arrested, but is released later when Rita comes to pick him up.
After he's freed, he finds out that Trinity is gone. He convinces Rita to go on vacation first, and he'll follow her there later in the day. He goes to confront Trinity in his house, but he finds out he's gone. A police SWAT team arrives unexpected, but Dexter manages to pretend he's working in the garage for forensics. He notices the shattered windscreen of Trinity's car, and realises that Trinity will go and collect his car later. He goes to the shop, removes the oilcap, and hides in the trunk.
When Trinity's car breaks down by the roadside, Dexter comes out and puts a sleeper hold on him. He then kills Trinity, and goes home, prepared to meet Rita for their vacation. However, a message arrives on his phone with a message from Rita. He calls her back, but realises her phone is in her bag in the house. Just then, his young son, Harrison, cries from the bathroom. He rushes in and sees Rita sitting in a bathtub crimson with blood. He picks up Harrison and rushes out of the bathroom, feeling guilty about Rita's death.
Guess that's about all the main points I can remember.
