Just as the Grateful Dead have been fairly and unfairly blamed for inspiring the aimless noodling and whimsical indulgences of jam-band progeny like Phish and Blues Traveler, so too has M. Night Shyamalan born the brunt of criticism for the blindsiding, 11th-hour twists popularized by his 1999 thriller The Sixth Sense.

Never mind that The Usual Suspects, with its own famously head-spinning denouement, predated Shyamalan’s breakthrough by five years. Every time a plot hinges on characters who don’t really exist (Secret Window, Identity) or some outlandish, deliberately misleading premise (The Village), it’s hard not to recall the moment we realized Bruce Willis was (spoiler alert!) just a sad-sack ghost in psychologist’s clothing.

You can forgive me, then, if The Alphabet Killer’s preposterous conclusion had me cursing M. Night’s oft-mispronounced name. Though everybody’s favorite punching-bag director (confirmed by thousands of CNN.com readers and a handful of critics, who ranked Shyamalan’s The Happening as last year’s worst film) had nothing to do with Rob Schmidt’s low-budget, quick-to-DVD thriller, his spirit looms large over a gimmicky ending that throws us for a loop.

That’s a shame, because The Alphabet Killer, which finds Eliza Dushku (of TV’s Buffy the Vampire Slayer) investigating a series of child murders loosely inspired by three real-life slayings that took place in the Rochester, NY, area during the early 1970s, takes us for a tense, unsettling ride until the wheels come off just 10 minutes before the closing credits.

Dushku, no stranger to tales of the supernatural, is all jittery nerves and spastic tics as Megan Paige, a schizophrenic investigator rapidly coming unglued as her search for a serial killer raises more questions than it answers. Someone is killing preteen girls with alliterative initials. Making matters worse, their decomposing corpses are crying out to Megan for justice as they appear before her in a series of ghoulish hallucinations. Rarely do they offer any helpful clues, but hey, that’s why it’s called a job.

In real life, the so-called Alphabet murders were never solved. Here, Schmidt (Wrong Turn) and screenwriter Tom Malloy, who also stars as Megan’s reluctant partner, single out a culprit. But in a movie featuring an ensemble cast of capable, easily recognizable character actors – among them, Cary Elwes, Timothy Hutton, Tom Noonan and frequent Rob Zombie collaborator Bill Moseley – it’s impossible to guess the killer’s identity. That’s part of the fun.

Not nearly as much fun is the unsatisfying endgame, which reveals the murderer to be… well, what kind of person would I be if I told you that? Suffice it to say that the killer could be anyone. Although Schmidt and Malloy have devoted meticulous attention to Megan’s manhunt, which is engrossing and briskly paced for the better part of 85 minutes, their final act seems less like a canny trick than a copout. The villain is chosen arbitrarily, ascribed no motive to kill much less exist in a story that unravels in the blink of an eye.

Why review a movie like The Alphabet Killer in the first place? After being released in just two New York-area theaters before heading straight for the video shelves, it’s one of those movies you’ve probably never heard of – I certainly hadn’t – that comes in a colorfully creepy package (see above) boasting a talent-rich cast. (Even Melissa Leo, a potential Oscar nominee for Frozen River, makes a brief but affecting cameo as a grieving mother.) And it’s far more cleverly executed than roughly half the movies that major studios spend millions to promote.

So why is it making what amounts to a perp walk into your neighborhood Blockbuster? Probably because actors like Dushku, Elwes and Hutton don’t qualify as marquee attractions, even if their work is of a consistently high caliber. And while The Alphabet Killer will hardly stand out as a featured entry on any of their Internet Movie Database résumés, it’s a scrappy whodunit that deserves a wider audience than recent remakes of Prom Night and The Texas Chainsaw Massacre.

Still, there’s that ending. Ever since Hitchcock exposed Norman Bates’ mother as the figment of a demented psyche, his disciples have sought new and exciting ways to lull their audiences into a sort of complacency, only to shock them out of it with a last-second surprise. Shyamalan’s trend-setting trickery has seemed increasingly less inspired since 2002’s Signs, though it’s worth noting that The Happening (which was not the year’s worst film but hardly one of its best) plays it more or less straight. As for The Alphabet Killer? It occupies the same unhallowed ground thanks to a payoff that’s unworthy of the buildup.

Elsewhere on DVD & Blu-Ray

Although most of its exclusive bonus features – including documentary-style studies of actual murder investigations and a podcast courtesy of starMichael C. Hall– will only be accessible via BD-Live after its Jan. 6 release date, it’s hardly premature to hail the Blu-Ray debut ofDexter: The Complete First Season as an unqualified success. Showtime’s riveting saga of a serial killer bound to a rigid code of ethics – don’t ask, just watch – benefits from a seamless transfer and rich dialogue whose clarity is neatly accentuated by a 5.1 Dolby TrueHD lossless track. The three-disc set features a pair of previously unreleased episode commentaries courtesy of the producers and cast, but even without the bells and whistles, Dexter on Blu-Ray remains a worthy investment if only for the chance to see the best show on television (now that The Wire is gone) at its most colorfully macabre.