Rita picks him up and is strangely okay with the whole thing. They decide to reconnect and have the honeymoon they never got to have. This works out well for Dexter, since he knows that Arthur is tracking him. The safest place for his family to be is not at their house.
Arthur then off to find Dexter's home address, but is foiled by bad record keeping in the post office when he goes to Deborah's apartment, which is still under Dexter's name. He then goes home and takes all his family's valuables hoping to flee while he still has a chance.
At the police department, they've figured out the ruse of the truck driver, and are able to nail down that the real killer is Arthur based on the testimony of the little boy who survived his abduction. A full commando swat team descends on Arthur's house while Dexter is questioning the family as to Arthur's whereabouts. The police cart the family away.
Deb also has some news for Dexter. She's been snooping around their dad's past and found out that Ms. Laura Moser was one of Harry's paramours. She also found out that, at the time of her death, Laura was survived by her two sons, Dexter and Brian. Deb knows that Brian was the ice truck killer, and figures out Dexter's true origins. She breaks the news to Dexter, and he does a convincing job of pretending not to know.
Dexter then descends into a pit of self-pity and worry for his family's safety, being that he's a serial killer and all. He doesn't want them to end up like the Mitchells, terrorized by their father and appalled when they find out the truth. But it's different! His family loves him, and he's not a jerk!
So, Arthur has definitely skipped town in his vintage Mustang, which he recently had repainted black. He rides off into the sunset with the police left scratching their heads. Will he get away with it all? NO! It's Dexter the Vigilante to the rescue. He'd somehow secreted himself into the backseat/trunk (it's unclear) of the tiny convertible and engineering a breakdown from said hiding spot. Dexter is magic, apparently.
And now, finally, FINALLY, Dexter can get to the business of taking care of Trinity once and for all. They have a heartfelt chat about the life of a serial killer, with Arthur repeating over and over "It's already done." Yeah, it's done...FOR YOU! Dexter kills him with the same hammer he used to kill that dude in the office building, just for old time's sake. He dumps the body, and is filled with a heretofore unknown sense of serenity, content that his family is safe and sound in the Florida Keys, and looking forward to joining them.
When he gets back to the house, he gets a message from Rita that she forgot her ID and had to catch a later flight. Oh well. He gives her a ring, but her cell phone and purse and sitting on the kitchen counter. Then, he hears Harrison cry. He follows the sound into the bathroom to find Harrison sitting on the floor, yowling and covered in blood. Rita has been killed in the bathtub, in the ritual way of all the young women Trinity kills.
So, wait, what? What's the chronology on this? Trinity leaves his house, goes to get his convertible, makes a pit stop at Dex's house (how did he find out it was his house if he was still listed at his apartment??), then rides off into the sunset (but not really)? Or was it SOMEONE ELSE?! Well, those questions, as important as they are, are now moot points. Dexter is now the sole custodian of three children, something that will certainly preclude his "working late" and "spending the night at the office". These kids need him, not some of the time, not when he's not dealing with the dark passenger, but always. At the end of the episode, before the Rita reveal, Dexter hoped and wished that his dark passenger would soon leave him free to a normal family life. Will he be able to repress it for his children's sake? Will he want to? So many questions! I want to know what happens! When is the next season?! So much excitement, but so much waiting.