Full Moon and Empty Arms : A DANGEROUS WOMAN, <i> By Mary McGarry Morris (Viking: $19.95; 360 pp.)</i>

If the sun or moon should doubt, they’d immediately go out, William Blake wrote; and he knew.

Yet, without cloud cover and eight hours’ darkness out of 24, the rest of us would go out with skin cancer, sleeplessness and eyestrain. And if the moon were full every night instead of only once a month, why, lacking the relief of waning, crescent and new moons, what a cataclysm of high tides, howling dogs, fuse-blowing passion, and--see the etymology--loonyness would afflict us all.

The dangerous woman of Mary McGarry Morris’ novel is always at full moon. In her troubled and partly unhinged mind, there can be no overcasts or gibbous phases.

The people in her small Vermont town regard her as a freak. Her freakiness shows itself in a series of monumentally obsessive attachments, and in an equally obsessive insistence on total, unwinking, unshaded justice. Short of monumental, nothing is real for her; monumental that crushes what it grabs. Her loves flee her and justice is repeatedly, brutally denied her.

From childhood, Martha Horgan has been excessive. In high school, she is big and ungainly; a full-figured allure is concealed by awkwardness and thick glasses. It is not just her looks that set her apart and make her suspect. She is a brooder, has a violent temper and directs hundreds of anonymous phone calls and unsigned love notes to Donny, the class hunk.

One night, some of the hunk’s friends lure her to a cabin in the woods, alleging that Donny wants to see her. At first, they all--hunk included--tease and joke. Martha smiles hopefully, taking it for affection and acceptance. Then they maul her, rip her clothes, fondle her sexually and flee, stranding her in the woods.

When she trudges home--her mother is dead and she lives with her father and a rich aunt--she is muddy, battered and distrait. The police are notified; the assailants are brought in and lectured. But it is the community consensus--her assertive aunt shares it; her father is dubious but timid--that the boys should not have their futures blighted for a prank that got out of hand. Besides, the townspeople say, Martha is peculiar and probably sex-mad.

Brutalized by judgment--far more than by the group-grope--she grows up as the town’s perilous odd one. She thinks of herself as a three-legged cow, the one that is regularly butted and bullied by the rest of the herd. It only makes her burn hotter.

She gets a job at the local dry cleaners where her fierce meticulousness puts everyone on edge. When the owner tries to cheat a customer, she makes a violent and embarrassing scene. When she sees the driver skimming the cash drawer, she denounces him, only to be charged with the shortage herself, and fired.

The agent of injustice is Birdy, her fellow worker, who had been kind to her and reaped the consequences. Martha is obsessively in love with her. Birdy betrays her to protect the driver, who is her lover.

Abused and drifting, Martha retreats back to the care of her aunt. A third passion inevitably develops: for Mackey, an alcoholic writer who has moved in to work as a handyman. He too has been briefly kind; he too will betray her, after responding briefly to her passion. The cycle of wretchedness will end in gory violence; another blind attempt to see too clearly, to wrest justice out of the denial of justice.

Morris, who plots and illustrates her cycle with a harsh and unstable brilliance that suggests her protagonist’s temper, sums it up at one point. Mackey tries to reason with Martha after some teen-agers splash her with mud and she goes wild:

“She looked at him. His face twisted with the same frustration she had seen in her father, in teachers who had all thought she should have some control over the fiasco that was her life. Of course she should. She wanted to, but did not, never had, and probably never would. And there seem to be no reasons for this, no mark they could see, no disfigurement or missing limb. One by one they would abandon her.

“That was the reason she always reacted so suddenly, loving so fiercely those who would be kind, so that in order to part they would have to tear themselves away from the ancient battlefield that was her heart, gouged and still, and survivorless.”

“A Dangerous Woman” is powerfully and dangerously written. To cast a blinding light on her protagonist, Morris has sacrificed subtleties or shadings. The balance of the outsider as both victim and helpless instigator of violence is precarious to maintain.

The townspeople and most of those with whom Martha tangles are virtually as one-dimensional as figures in a parable. Morris assigns their roles and runs them with an iron hand. Birdy, the treacherous friend--but who could be faithful to such bottomless claims?--is barely seen. Mackey and Martha’s aunt--they become lovers, thereby betraying her in still one more way--are denser and more complex. In the main, though, they live in terms of Martha’s panicky and unbridled apprehensions.

The author has risked this along with the rough and sometimes disjointed tone of the book for a single purpose. Her Martha, Gothic excesses aside--or perhaps not aside--is memorable. The reader is likely to shift between finding her a monster of excess, like the woman in Stephen King’s “Misery,” and a pure innocent. This accounts, incidentally, for some of the feeling of disjointment.

In fact, though, she is neither, and both. I am not sure exactly what larger questions about the individual in society are aimed at. But “A Dangerous Woman” is an indelible evocation of three-leggedness as a human as well as bovine condition. The ending, profoundly touching, joins us to Martha in a fashion that is the more compelling because, throughout the book, we had been uneasily coming close to her and shying away.