A simple plot summary of Sue Miller's "The Senator's Wife" makes the novel sound like a cross between a Lifetime Channel movie and a parody of an Oprah's Book Club selection:

Two women, two generations -- and one wall between them. When thirtysomething Meri moves into one side of the duplex in a New England college town in 1993, she meets seventysomething Delia, living on the other side. Meri (new wife of an ambitious young professor) turns to Delia (decades into her unconventional marriage to a womanizing, Kennedy-era politician) for the maternal role model she never had. Secrets, lies and a shocking betrayal follow, transforming Meri and Delia's lives in ways they never could have imagined.

Yes, that is what happens. But it's a woefully inadequate expression of the masterful style and substance of this fine book: a rich, elegantly plotted tale of two women's -- and two generations' -- experiences of marriage and motherhood.

Miller's success began in 1986 with "The Good Mother"; in her eighth novel, she's at the top of her game. Miller herself is in her 60s now, and this tale -- of pregnancy and early motherhood on one side of the wall, of growing old on the other -- reads like the reflections of a wise, shrewd and compassionate woman, herself poised demographically between the maternity ward and the nursing home. But again I'm drifting toward Lifetime-ish platitudes, and there's nothing easily sentimental or reassuring about what Meri and Delia discover about themselves.

In the generational point-counterpoint of "The Senator's Wife," Delia is a post-feminist anachronism: the campaign helpmeet in suit and pearls, smiling through the (private) serial infidelities of her husband and standing (in public) by her man. Though still "married," Delia and Tom haven't lived together for decades.

But when Meri, gripped by "an appetite ... to know more, and then more than that, about Delia's life," goes snooping in her neighbor's house, she finds passionate love letters from husband to wife over the years, and later she hears Tom's voice in Delia's bedroom.

"For a woman in her seventies ... [Delia] looked absurdly glamorous." (Helen Mirren would be a shoo-in for a movie version.) Taken inside Delia's thoughts, we discover both a hard-edged survivor and a romantic dreamer.

It would seem that Delia has nothing to teach thoroughly modern Meri, who is confident that Delia and Tom's "sort of old-fashioned and kind of unequal romance of a marriage" bears no resemblance to her own, more liberated, marital partnership. But Meri becomes unexpectedly pregnant. Home from a harrowing birth experience with a fussy infant and a raging case of post-partum depression, Meri finds herself in a role strangely parallel to that of Delia, who also unexpectedly finds herself a caregiver -- at the other end of the aging spectrum. "The Senator's Wife" provocatively invites us to ponder: Have women's roles changed over the past 30 years as much as we might wish to believe?

I love the way Miller allows both her heroines to live fully inside their bodies at different ends of the generational spectrum, from morning sickness to arthritic aches. Whether 35 or 75, these are flesh-and-blood women with sensuous appetites and sexual desires. Psychologically, they're richly detailed, flawed humans. Meri revels in the power that small secrets and lies give her over her husband; Delia selfishly guards her emotional resources -- she "was aware of feeling that Meri was asking something of her, and she was aware of her own resistance to that."

"The Senator's Wife" could make a wonderful movie, but I'd urge you to read the book first. Like the recently movie-ized "Atonement," it unpacks surprises that work best on the page: two unexpected, revelatory narrative time-shifts (one backward and one forward), and a plot development at the end that deftly manages to feel both utterly shocking and completely inevitable.

It's a measure of Miller's brilliance that I'm still thinking about the implications of the way she concludes her book. It's intentionally ambiguous: both bright and dark, celebratory and desolate. Nothing at all like a Lifetime Channel movie, but a lot like real life.

Diana Postlethwaite is a professor of English at St. Olaf College in Northfield, Minn.