With his trademark understated perversity, Alfred Hitchcock once declared that his goal as a filmmaker was to bring murder back into the home where it belongs.

And before their sophomore effort wimps out, writer-directors Scott McGehee and David Siegel succeed in doing just that, transforming the bourgeois household into a constricting cage of suspicion, lies and dread. “The Deep End” belongs to that class of noir where the best motives drag otherwise decent characters into the lurking moral netherworld. It’s the simple, noble urges that drive such sympathetic figures to murder and deceit.

And who better to draw audience sympathy than Margaret (Tilda Swinton), whose once lovely face has been left taught and drawn by the worries of caring for three children while her naval officer husband is away at sea. She’s a dedicated mother, worrying about the inner turmoil of her 17-year-old son as he copes with his emergent homosexuality, and about the reaction of her yet unsuspecting husband.

Margaret has good cause for concern. Her son’s lover is a 30-year-old nightclub owner who is perfectly willing to leave the boy alone – if his mother pays him enough. And when her son’s sleazy paramour shows up dead on the beach behind their Lake Tahoe home, she only wants to protect her son. Yet even the corpse doesn’t prepare her for the sinister young man (Goran Visnjic) who appears on her doorstep.

The house by the lake proves a rich visual metaphor for the film’s thematic concerns. Domesticity sits perched on the edge of an eerie, almost seductively beautiful expanse of water, hinting at the moral abyss beneath. The filmmakers return time and time again to the motif of drowning, with shots of an indoor pool or a child’s fishtank providing sinister undertones in otherwise benign contexts.

The house itself begins to hint at the trap in which Margaret finds herself. Viewed from the outside, the house’s row of picture windows suggests a terrarium, as if Margaret and her family were mere rats in the maze of a sadistic researcher. The effect is far more unsettling than the traditional creaking of stairs or ominous shadows it replaces.

If the film’s plot set-up seems like a typical noir contrivance, the motivations of its central characters do not. Swinton’s performance carries the film by virtue of her restraint as an actress. Rather than lapsing into oscar baiting histrionics, she maintains an air of cool elegance, even as her eyes suggest a struggle for composure in the face of unbearable stress. Smart, composed and careworn, she’s the most convincing cinematic mother in years.

As her troubled son, Jonathan Tucker gives an equally understated incarnation of simultaneous intelligence and youthful naivete. He’s appealing enough to make his mother’s efforts to save him resonate with the audience, and ambiguous enough to allow both mother and viewer to wonder what he’ s capable of.

With so much going for it, “The Deep End” is both startling and disheartening when the film goes thoroughly awry about halfway through. Having meticulously tightened the screws of Margaret’s predicament, the screenwriters simply allow the suspense to ease. Once the trap has been sprung, the rest of the film never recovers its hold.

The slackness of mood is accompanied by a still more dismaying slackness of plot, in contrast to the narrative and visual precision of the film’s first half. One of the delights of such a carefully crafted film is watching each plot element click effortlessly into place, providing both visceral tension and a subtler aesthetic pleasure. Perhaps “The Deep End”‘s most disappointing feature is that so many of its plot elements ultimately prove extraneous.

While the non-mechanical nature of its plotting does do something for its stature as drama, giving it an emotional resonance beyond formal elegance, it also allows the film to lose its grip. The film’s conclusion is moving, but nonetheless cries out for the malevolent efficiency that gives a film like “Memento” its punch.

If McGehee and Siegel had finished the film in accordance with the chilly fatalism of its beginning, “The Deep End” might have been the best movie of the year. Unfortunately, they didn’t dare to take the plunge.