There is nothing more depressing than watching a good movie fall to pieces with a sudden thwack! to the side of the head, especially a movie you’d anticipated beforehand was going to be pretty dreadful. And who wouldn’t have thought “John Tucker Must Die” was going to be insanely awful? Those intolerable trailers and commercials were easily some of the year’s worst, and the last thing I figured the world needed was another high school comedy running in the same vindictively observational vein of “Mean Girls.”

Boy was I wrong. Director Betty Thomas (“The Brady Bunch Movie,” “Private Parts”) certainly returns to form with this smart, witty, sexy and extremely funny comedy. Television sitcom veteran Jeff Lowell’s (“The Drew Carey Show,” “Cybil”) screenplay strikes far more hits than it does misses. In fact, the script goes out of its way to create three dimensional characters actually worth the time getting to know. Like “Clueless” and “Mean Girls,” sitting in the theater I quickly realized “John Tucker Must Die” was on its way to becoming a sensational surprise.

And then comes that climax quickly turning a delightful, energetic romp into a “10 Things I Hate About You” style disappointment. Much like that 1999 secondary school Shakespeare adaptation, the filmmakers here haven’t the first clue as to how to bring things to a reasonably satisfying conclusion. The third act is jaw dropping in its awfulness, the film ending on a coda so turgidly lifeless I couldn’t help but scratch my head wondering what the flying heck happened.

Still, for a good hour or so this thing clicks on so many cylinders General Motors would probably like to know the filmmakers’ secrets. From pacing to acting, editing to cinematography, Thomas and her team craft a comedy I couldn’t but help fall in love with. While there’s nothing new going on, the whole enterprise is so pointed, so bouncily intoxicating the familiarity of it all never mattered near as much as it probably should have.

New girl Kate (Brittany Snow, “The Pacifier”) is enlisted by three popular students; head cheerleader Heather (Ashanti, “Coach Carter”), wannabe investigative journalist Carrie (Arielle Kebbel, “Be Cool”), vegan animal activist Beth (Sophia Bush, WB’s “One Tree Hill”); to help them takedown their school’s unquestioned superstar John Tucker (Jesse Metcalfe, ABC’s “Desperate Housewives”). He’s been dating all three girls all at the same time and now they want revenge, and the timid-yet-beautiful Kate is the key to making the smarmy philanderer pay.

It’s all a bit more complicated then that, but not too much so. Kate has her own insecure demons of running around from school to school thanks to unlucky in love mother Lori (a shockingly winning Jenny McCarthy, “Scary Movie 3”), and John’s alternative leaning brother Scott (Penn Badgley) is thrown into the mix as a potential love interest. Other than that, though, this film is as thin as a vanilla wafer, and those expecting something a bit meatier or more complicated had better turn their attentions elsewhere.

But a film doesn’t have to be complicated or be about big all encompassing seismic cultural issues to be entertaining, and for the majority of this one’s running time it certainly is that. All four actresses are a delightful hoot, Bush and Snow especially winning crafting characters far more intoxicating than I’d ever have expected them to be. They’re both borderline fantastic, and while one of their moments is a sure bet to pop up as a MTV Movie Awards contender, it’s the fact they create flesh and blood people worth caring about that got me excited.

And then it all comes crashing down with a resounding thud. Thomas and Lowell haven’t the first clue as what do with it all. This messy climax is as inept as any I’ve seen this year. What should have been a high school comedy winner instead becomes a high school comedy downer, a clunky disappointment that sent my out of the theater annoyed and disheartened by how the filmmakers wasted such wondrous potential.

Still, I can’t be too harsh. For a while there “John Tucker Must Die” had me wrapped around its little well-manicured finger. It’s funny, smart and acted with remarkable dexterity by the four leads. Better, the humor in the whole thing just kills. I just wish the climax didn’t, much like what the women want to have happen to the title character, die.