IT'S ONLY a TV show.

That's what I was telling one of my editors yesterday morning about Sunday's "Dexter" season finale.

If you're one of the viewers who were more than passingly disturbed by the, um, killer ending, I'm telling you the same.

And if you still have "Dexter" waiting on your DVR or Showtime On Demand queue, please stop reading now, or I really can't be answerable for the consequences. You have as long as it takes to read this sentence to turn the page or skip ahead to the next link.

OK, everyone up to speed now?

Yes, that was Rita (Julie Benz) whom Dexter (Michael C. Hall) discovered in the bloody bathtub when he returned home after disposing of the Trinity Killer (John Lithgow).

Yes, Arthur "Trinity" Mitchell is almost certainly responsible.

No, it was not a dream, as Entertainment Weekly's Ken Tucker discovered some of his readers believed after seeing the episode.

I like to believe Daily News readers are too savvy to fall prey to that kind of magical thinking, but I'm wondering how many of you were surprised Rita didn't make it out alive.

Because I'd have been shocked to see her still breathing in Season 5 (not that being dead has stopped James Remar's Harry Morgan from popping up every week).

After all, "Dexter" writers had done everything short of drawing a target on Rita's chest this season. Plus, persistent talk of a "game-changing" finale pointed to a shift in Dexter's circumstances that would likely derail his progress toward becoming a happier, and presumably less lethal, human being.

That kind of progress can be a problem when you're making a show about a serial killer, however sympathetic he's meant to be.

If you look at the arc of Dexter's experience with Rita, whose transition from clueless girlfriend to clueless wife was handled so well it became possible to believe Dexter might actually love her, you can see why the writers wanted her dead.

She was cramping Dexter's style.

But what of baby Harrison, who was presumably plopped into the bloody bathroom scene by Trinity in what looked like a re-creation of a pivotal experience in Dexter's early life?

The image is so perfect I can imagine it pinned to a board on the writers' room wall as a sort of goalpost for the season.

Maybe it's a little too perfect, though.

For one thing, given how much less Trinity knew about Dexter than Dexter knew about Trinity, why would he have bothered to place the baby in the scene at all? Even Debra Morgan (Jennifer Carpenter), a pretty good detective, took years to discover that bloody bit of her brother's past.

I can get past the fuzzy timeline for Trinity's final hours of freedom and the apparent gaps in Dexter's own day, but I'm not sure I can get past that.

It's been a good season, nevertheless, for "Dexter" and especially for Carpenter, whose Debra remains the show's second most interesting character, and perhaps the one enduring love of his life. (And I'm not just saying that because Hall and Carpenter are married.) Thanks to her, the death of FBI agent Frank Lundy (Keith Carradine) continues to resonate.

Dexter's future as a single father wouldn't seem to promise more free time for his extracurricular activities, but then watching him navigate the shoals of so-called regular life has always been part of "Dexter's" appeal.

I wouldn't, however, want to be his baby sitter. *

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