“The messes are piling up,” ghost-Harry warned Dexter in Sunday night’s season-four finale.

“You have your demons, I accept that,” said Rita to Dexter.

“Nothing is inevitable,” said Dexter to Arthur Mitchell, the Trinity killer.

I’m telling you right now, do not read this unless you’ve seen the shocking Dexter season finale.One more time: SPOILER ALERT!

Okay. Who saw that ending coming? I didn’t. And, yes, I remember that I’m the guy who was complaining about what a whiny pain in the butt Rita was becoming earlier on in the season.

But it says everything about the excellence of Dexter that the season not only gave us a completely satisfying, firm conclusion to John Lithgow’s superlative run as this season’s chief villain, but also sent the series spinning into a whole new direction for next season.

First: The hour began right where last week left off, with Dexter confronted by Arthur in the police station. Pretty soon, we saw a Dexter typical of this season: pressured, pulled in a number of directions, distracted and off his game. Having decided that Arthur had to die and fast, he began a wild car chase (“Do you even have a plan here?” said the Harry at his shoulder — bravo to James Remar for excellently understated work all season). And of course, in the midst of the chase, Rita called our guy, which led to dinging an innocent citizen’s car, which led to Dexter losing his temper with some investigating cops just after Dexter thought he had Arthur trapped where he wanted him.

“I didn’t follow the code — I lost it,” Dexter berated himself.

Meanwhile, Deb finally discovered that Dexter and the Ice Truck Killer were brothers. And congratulations to Jennifer Carpenter for a performance all the more remarkable this season for the fact that she had to play so many scenes of extreme emotion — cycling through falling in love with Lundy all over again, witnessing his death, suffering survivor-guilt over that, and now… this — and Carpenter did it without repeating any of her reactions, making Debra experience each fresh pain in a different, beautifully distinct manner.

The episode was building to the moment we’d waited for all season: Arthur on Dexter’s killing table. I cheered at the screen when Dexter slammed the hood of the Mustang down on Trinity’s head and said with reptile warmth, “Hello, Arthur Mitchell,” making ironic fun of the way Mitchell had greeted him.

Lithgow got to play his final scene immobile, strapped down, but fully capable of making us believe that, even at his most physically helpless, Arthur’s psychotic confidence enabled him to take command. He lectured, trying to deny Dexter a pure triumph, saying, “I was following my path. God led you to me.”

And once Dexter had dispatched Arthur — with the sharp end of a hammer, an impeccable touch — we got the Dexter ending we’d wanted: Our man dumping Trinity’s body over the side of his boat, and a shot of Dexter raising his head in vast relief, framed against a blood-red night sky.

And then we got the ending we didn’t see coming. The fearful symmetry of Dexter finding his son Harrison sitting in a pool of blood (“Born in blood, both of us,” in Dexter’s voiceover phrase), and Rita murdered in the bathtub.

“I’m what’s wrong — this is fate,” said Dexter.

So now Dexter had to take back all the brimming optimism he was nurturing within himself just a few minutes before. What does this mean for next season?

Did you stick around for the chat Michael C. Hall and John Lithgow had after the credits rolled? They were right: This was an ending designed to “tie people in knots.” It could indeed, as Hall suggested, cause Dexter to shut down emotionally, believing that it’s best if a man with a “dark passenger” to have room for no one else in his life. I suppose those grandparents we saw taking Rita’s kids off to Disneyworld could take them in, but… oh, it’s too soon to speculate. I’m just going to revel in the smashingly great way the season ended. Looking at the season as a whole now, Julie Benz deserves a lot of credit for playing Rita with such selflessness, risking losing some of our sympathy to make the final scenes all the more powerful.

Cheers to Michael C. Hall for letting us see new, vulnerable sides of Dexter without ever losing the witty, slit-eyed menace that makes the character so compelling, and to the producers and writers for maintaining the suspense while creating a whole new world within the Dexter world: the awful universe of Trinity and his own trapped family.

Traps are what Dexter is all about: How to get in them, out of them, and still realize there’s another you cannot predict.

What did you think of the Dexter finale?

More Dexter from EW:

Exclusive: Dexter exec producer Clyde Phillips talks finale with Michael Ausiello