A DANGEROUS WOMANBy Mary McGarry Morris Viking. 358 pp. $19.95

The epithet"a dangerous man" makes us think of a comic-book desperado with a weapon in his hand, but "a dangerous woman" conjures a comic-book decolletage: We see a woman who carries not a weapon but her powerful sexuality out in the open. For in women, not violence but sex without shame is most dangerous to men, to other women and to the received order of the world.

And so I hoped that "A Dangerous Woman," Mary McGarry Morris's second novel, would be, like Lynne McCall's recent "The One True Story of the World" and the books of Margaret Diehl and Austrian novelist Elfriede Jelinek, a new exemplar of the feminist novel in its splendid decadence, in which women smart enough to know better pursue strange adventures because no one man is ever enough.

But the heroine of "A Dangerous Woman" is hardly too savvy for the world she's stuck in; in fact, Morris must assure us repeatedly that Martha Horgan is not retarded. Wholly unrequited people interest Morris, like the hapless, doomed kidnapper Aubrey Wallace of her first novel, "Vanished," and now Martha. Motherless from babyhood, isolated by her own strangeness, Martha sees herself as a literal blank:

"She could almost feel it, that huge blank echoing space where nothing, not even the beat of a heart, could regulate all the turmoil that found its way inside."

Martha Horgan is mentally ill: nearly unemployable, lacking in worldly judgment, rigid about rules and rituals, socially inept but not submissive to ridicule; indeed, easily provoked to violence, especially against children, whom she hates. Full-breasted with a pretty face, she is plagued by tics and a choking sensation that causes her to thump her chest compulsively three times.

Martha does outrage the Vermont town of Atkinson by wearing her sexuality in the open, not because she has confidence in it, but because she doesn't know how to put it away. She is subject to ferocious attachments, and if her crush is on a man, Martha is almost unhinged by lust. When she is smitten with the hired man Colin Mackey, her aunt Frances snaps at her, "For Godssakes, don't ever stare at the front of a man's pants like that!" And Martha knows it is true: "Loving him was all she ever thought of now."

Sometimes Morris plods. The novel's opening sentence foreshadows a murder at the end of the book, and there follows a scene of stock outrage -- at 16, Martha is molested in the woods by boys her own age -- to explain what doesn't need explaining. But once the main plot is underway, the novel has power. Martha is fired from her job at a dry cleaner and obsessively tries to telephone Birdy, another counter clerk who was briefly nice to her. Meanwhile she returns to rooms she once shared with her late father on her Aunt Frances' estate.

Frances Beecham, widow of a financier 40 years her senior for whom Martha's father was caretaker, rails at Martha to practice self-control even as her own affairs slide into disorder. Her lawyer boyfriend of many years is about to return to his alcoholic wife, and Frances feels the downward suction of her debased kinfolk, "the dirt-poor Horgans" and Herebondes of "the Flatts." Her sense of the nearness of the dirt they both came from is of course what scares her about Martha.

Frances is a wealthy woman, but that "sense of uneasy tenancy" makes her stingy in the care of her estate, so that she hires a down-and-out writer as handyman. And he, Colin Mackey, or Mack, in two brief, tortured, drunken episodes that are well handled by McGarry, becomes Martha's first and only lover. Not without conscience, he soon realizes he has toyed with a damaged person and tries to back away; at the same time he cynically becomes Frances' lover. But Martha listens to Frances and Mack making love and makes love to herself with them.

At her best in creating jagged, tumbling imagery for the thoughts of her protagonist, Morris sometimes lectures us about lovelessness and its pathology in clumsy prose. She can be comic-book obvious, as with the town undertaker who courts Martha. But Morris has created a remarkable portrait of a disturbed woman who is a fully sexual creature. Her book raises the interesting question whether sexual love, even partial, fitful, throwaway love, is not better for Martha Horgan than no love at all. After all, some version of that defective love is what most of us get. The reviewer's most recent novel is "She Drove Without Stopping."