What separates a simple and familiar tale of rejection and bloody retribution from a thoughtful, character-based horror film? What turns a tale that could easily be run-of-the-mill exploitation into a sincere, heartfelt and appropriately creepy and fascinating film?

In the case of May, an engrossing little horror picture that plays like an unwieldy pastiche of genre films somehow fused into a startlingly sincere and affecting creep show - the deciding factor has much to do with the revelatory performance of a dedicated young actress determined to create a multi-dimensional anti-heroine who lives, breathes and surprises us from scene to scene.

As a child, May Canady has problems right from the start - born with a "lazy eye" and forced to wear a patch at school ("Are you a pirate?"), withdrawn little May has no friends - until one lonely birthday her disturbed mother delivers the most terrifying-looking doll you might ever lay eyes on, appropriately encased in a glass box, as if a refugee from a wax-museum freak show. Best friends, they fast become.

Flash forward to a now grown May (Angela Bettis), who works as a veterinary assistant and is the epitome of a shy, withdrawn and lonely young woman. She's the kind you pass on the street everyday and assume to live single, with a cat and a television, few romantic prospects and resigned to a quiet life alone.

Everything changes for May with the arrival of a major crush on cute and charismatic mechanic Adam (Jeremy Sisto, bearing more than a passing resemblance to a young Travolta), and the emergence of a lipstick lesbian office-mate, the energetic and sexually aggressive Polly (Anna Faris, flamboyantly flashy).

The bulk of the film is spent on May's gradual reaching out, attempting to trust and forming doomed relationships with this pair. When both engage May then almost simultaneously pull away (for understandable reasons), their decisions ultimately lead to disastrous results. If you've seen enough of these films to know where it might be going, May's already-fragile world comes dramatically crashing down, in a series of painful scenes that would be at home in any fine adolescent drama about the pain of not fitting in.

But then horror elements, never far from surface, begin to bubble to the fore. There's May's creepy dependence on her only confidante, that awful (and seemingly alive) doll. There's her gleeful fascination with the kinky (read: bloody) extremes of her relationship with Adam. There's her almost too-willing craving for a desperately experimental lesbian encounter. And then there's the fascination with sewing and amputation, which leads her to the film's most gruesome moments, all handled in a manner so straightforward and not trumped up, that you can feel them coming directly from May herself instead of from the necessity to turn up the film with cheap suspense.

There are countless memorable moments in May: May's startled reaction upon overhearing Adam's true feelings; the life drifting from a victim's eyes as the blood matter-of-factly drips from a casually slashed throat; May and a classroom of blind school kids awash in glass fragments and blood on a classroom floor; May unexpectedly kissing Adam's hands when they part.

The less said about the plot the better, but suffice to say that by its final reel, May is a full-tilt horror film, with a tongue-in-cheek climax staged on Halloween night, which perfectly blends horror and pathos in equal measures.

Written and directed with heart and smarts by Lucky McKee, May probably has no right to be as good as it is, but in the hands of the astonishing Bettis, it becomes a complex symphony of rejection, loss and humiliation - not unlike a certain classic horror film about another shy, withdrawn young woman that introduced Sissy Spacek to the world nearly 20 years ago (and which recently starred Bettis in a television remake of late).

Bettis, with her uncanny resemblance to a young Holly Hunter (and with the same sort of crusading intelligence), pulls off a nifty trick: she makes you care deeply about May, and with great empathy, while keeping you at arm's length, never letting you forget how deeply disturbed she really is. It's a tricky performance with equal measures heart and horror.

And though May bears resemblance to many other horror films, from Brian DePalma's elegant Carrie to William Lustig's notorious Maniac (at least in their concluding scenes), May benefits tremendously from Bettis' full conviction in the role, which just about elevates the film from its genre.

The film ends on a poetic and pathetic note that reminds us again how May's real intention is not the horror, per se - it's the humanity born from the horror.