When you're "The Simpsons" and you've been around for 27 seasons, and close to 600 episodes, doing something new is almost impossible. For the most part, the show's better latter-day episodes(*) mix and match stories the show has done a half dozen times or more in the past, finding amusing and/or poignant variations on old themes. Last week's Maggie subplot, for instance, featuring the triumphant return of Spider-Pig, had more than a little in common with the Oscar-nominated "Simpsons" short "The Longest Daycare," while the delightful "Cue Detective" episode from two weeks ago was cooked up from ingredients used in many past food-related episodes.

(*) And, yes, the series is still capable of doing very good, and at times, great, episodes even if it's not with the frequency of the first decade. We picked out some of the better recent installments in the later installments of our guide to FXX's Every "Simpsons" Ever marathon.

This Sunday night's episode, however, is remarkable, in that it finds a way to do something "The Simpsons" has never done before:

A real Halloween episode.

The "Treehouse of Horror" trilogies have been a "Simpsons" tradition going all the way back to season 2, but they explicitly take place outside of continuity, allowing for Homer's head to be turned into a donut, or Lisa and Bart to gain super powers to fight evil alongside Lucy Lawless, or alien invader Kang to be elected president because Americans won't vote for a third party candidate.

"Halloween of Horror," on the other hand, is a regular episode of the show that just happens to take place on Halloween. Written by Carolyn Omine and directed by Mike B. Anderson, it's primarily a Lisa and Homer story, where Lisa's so traumatized by her first visit to Krustyland's annual Halloween Horror Night that the family has to take down its traditional "Everscream Terrors" lawn display of spooky holiday decor. Bart is dismayed, and Homer is just plain worried, since his attempt to give his little Lisa a quiet and reassuring October 31 is disrupted by a trio of vengeful "pop-up scum," whom Homer accidentally got fired from Apu's pop-up Halloween store.

There are elements of "Straw Dogs," slasher movies, and even "The Rocky Horror Picture Show" — with a musical number where Bart unfortunately learns how adults celebrate the holiday — and all of it actually feels creepier than a typical "Treehouse of Horror" story because it's "real." No, nothing bad is actually going to happen to Lisa or Homer, but the creative team and Yeardley Smith make you feel her terror much more acutely than in the Grand Guignol style of the "Treehouse" tales. And like in an actual horror movie, the laughs become a more welcome tonic because the peril seems genuine.

There are, as you might expect, various winks to the show's usual Halloween tradition, including a meta exchange between Homer and Flanders about how "Treehouse of Horror XXVI" will be airing next week, but for the most part, "Halloween of Horror" plays things straight. Even with the enduring popularity of the "Treehouse" episodes, it's amazing the show's never tried a more traditional Halloween episode before. There have been many episodes set on Christmas, Thanksgiving, Valentine's Day, Easter, and Independence Day, not to mention "Simpsons"-only holidays like Whacking Day and Love Day. October 31st certainly didn't lack for coverage on the show, but this approach is one they for some reason never tried before.

And it works very very well.

Alan Sepinwall may be reached at sepinwall@hitfix.com

Alan Sepinwall has been reviewing television since the mid-'90s, first for Tony Soprano's hometown paper, The Star-Ledger, and now for HitFix. His new book, "TV (The Book)" about the 100 greatest shows of all time, is available now. He can be reached at sepinwall@hitfix.com