"American Beauty," a murder mystery that mutates into a scalding satire of the suburban myth, tallies up the staggering emotional cost of maintaining the manicured, middle-class ideal. It's a triumph--adult, smart, involving, stylish. Unlike so many of this summer's movies, it will make you think about the way you lead your life.

It's also a real estate broker's worst nightmare, demonstrating once and for all that mortgaging one's soul is never worth the return on the investment--not that Lester Burnham (Kevin Spacey in a bravura performance) will be making any more payments.

Alas, Lester can no longer benefit from the film's unsettling insights because he's already dead. "Of course, I don't know that yet," observes Lester, whose off-screen gibes and keen observations pepper this look at the last chapter of his life. From the sound of it, he doesn't miss it very much. And with good reason.

As dysfunctional-family films--from 1967's "The Graduate" to last year's "Happiness"--have illustrated so deftly, there's something rotten in Reston. Father doesn't know best anymore, Princess is a drug addict and Mom couldn't care less about shiny linoleum floors.

"American Beauty" plows the same turf, but its scenario is as unpredictable as its characters are provocative. It's unknown territory for English theatrical director Sam Mendes ("Cabaret") and TV sitcom writer Alan Ball ("Cybill"), both making their silver-screen debuts. They bring not only fresh perspectives but also unexpected warmth and sincerity to this prickly, richly layered tale.

The film begins with an aerial overview of Lester's nondescript home town, his street and finally his house with its white picket fence and carpet of fescue. His wife, Carolyn (Annette Bening, also in bravura mode), prissily tends her perfect rosebushes. "See the way the handle on those pruning shears matches her gardening clogs? That's no accident," says Lester, who describes his marriage to the frigid, flighty Carolyn as "a commercial for how normal we are when we're anything but."

Although Carolyn refuses to accept it, their love has faded, their marriage is a sham, and, like her husband, she is withering inside. Their sullen teenage daughter, Jane (Thora Birch), is fed up with both of them but finds solace in her friendship with Angela (Mena Suvari), an all-American blond beauty who is as wild as Jane is reserved.

Although Jane is accustomed to her friend's effect on men, she didn't expect her father to begin slobbering over Angela, who playfully encourages Lester's attention. "Could he be any more pathetic?" Jane asks in disgust, but Angela enjoys the attention and hints that if he were in better shape, she'd definitely be interested.

With that, Lester is reborn. He digs out his old weight-lifting equipment and begins to pump iron. As he grows physically stronger, he starts to examine his whole life: the unsatisfying job, the unappreciative family and the rigidity of his home life. In an unusually gutsy move, he not only quits his job but blackmails his employer into giving him a generous severance package and becomes as rebellious as a teen.

Meanwhile, mother and daughter find outlets, respectively, in the pompous king of local real estate (Peter Gallagher) and in Ricky Fitts (Wes Bentley), a voyeuristic teen who never goes anywhere without his video camera. He records everything, though his favorite subject these days is Jane, who rather likes being the center of attention for a change.

Ricky's parents are even more troubled than Jane's. His father (Chris Cooper) is a homophobic Marine colonel who slaps his son around; his sad-eyed mother (Allison Janney) is virtually catatonic. Jim and Jim (Scott Bakula and Sam Robards), a gay couple, are the only functional family in this deceptively cheery neighborhood.

Aside from the Jims, the characters don't seem likable, yet they are so complexly drawn and pitifully human that we can't help ourselves; we like them anyway. And they are so nakedly acted by this universally splendid cast that we come away close friends. Bening is so convincing as this brittle Barbie with the tight smile that she seems in danger of shattering all movie long.

Bentley, as the teenage peeping Tom, is intense, mysterious and a little scary. He seems to haunt the community, and Birch's misery and hidden hope make her Jane a perfect partner. And Suvari is perfectly cast as the cheerleading Lolita.

But Spacey powers the movie, capturing Lester's stifled spirit, his wounded manhood and his imperiled soul. It's hard to believe, but Spacey, who is best known for his dark roles like the killer in "Seven," is not only sympathetic as the rebellious suburbanite but also very vulnerable.

Though nobody ever promised us a rose garden, "American Beauty" bears blossoms along with its thorns. And though we can't smell their perfume, the film is utterly intoxicating.

American Beauty (118 minutes, at area theaters) is rated R for sexuality, language, brief nudity, violence and drug use.

CAPTION: Annette Bening as the prissy, garden-variety wife in "American Beauty."