They Didn’t See It Coming : UNTIL YOUR HEART STOPS By T.M. McNally , (Villard: $20; 320 pp.)

It is a warm October evening in Scottsdale, Ariz. Outside the high school gym, a few students and teachers are waiting for rides. Walker, a polite boy with big feet, walks past two friends, Edith and Howie, and the swimming coach, Jenna Williams. He sits down 20 feet away, opens a bundle he is carrying and pulls out a pistol. Howie starts slowly toward him.

“No. I’m sorry,” Walker says; and then, as Jenna and Howie remonstrate: “Don’t look. I’m sorry. Just don’t look, then.” He kills himself with a bullet through the eye.

Walker’s suicide and the currents that lead to it, eddy around it and flow out from it are the material of T.M. McNally’s powerfully and finely delineated novel, “Until Your Heart Stops.” Its theme goes beyond these things to cast a cold light on the disarray of adults who were young in the ‘60s, and on the moral and existential wilderness bequeathed to their children, now approaching adulthood themselves.

McNally, a first novelist, handles his theme with a certain degree of awkward rhetoric and a story which, in the events that follow Walker’s death, leans too strongly on dire melodrama. The book has several shining strengths; mainly, in the formidably compassionate way in which it evokes the shifting states of mind of Walker, Edith, Howie and a fourth friend, Joe.

The author has an inspired vision of the latency of adolescent character and perception. He uses a technique that is both complex and thrilling to give a sense--as with a photograph developing before our eyes--of the movement between what each of his young people knows and what they are coming to know. His work is less inspired, or perhaps less original, with the adults--Jenna and Ray, the wrestling coach--who are the two other main characters.

For the four adolescents, all victims in one way or another of what McNally sees as the Saturn complex of our times--the older generation’s freedoms and indulgences have consumed its progeny--everything is simultaneous. They are not yet able to arrange in linear and hierarchical patterns the data around them. They experience their wilderness without paths.

And so, in the first section, we see Joe going about his day. He works out rigorously for his demanding coach, Ray. He does assignments for a demanding and brilliant writing teacher. He thinks of Edith, his girlfriend. In his father’s frequent absences on business, he takes care of his younger brother, Spencer, healing slowly after being hit by a car.

Only casually do we get notice of the chasms in Joe’s active and generous-spirited existence. Walker, we learn--long before the event is described--has killed himself. Walker had spent time in a sanitarium after a breakdown. Walker’s mother had committed suicide a year earlier. And Joe’s mother ran off to Portugal and they have not talked for two years.

With Edith, the book’s most touchingly memorable character, the big terrors of past and present are similarly squirreled away in an adolescent’s system of power surges and protective cut-outs. She has been near-blind from birth; operations have given her a borderline state of legal blindness and the privilege of using contact lenses three times a week. She insists on taking driver’s ed, gets perfect written scores, and only bashes an occasional fender in practice. (Spencer, in a wheelchair, is determined to ski.)

Bit by bit we get the submerged pain. Her family has moved to Arizona to make a new start. Her father has promised to stop doing “those things,” and her mother has agreed to stay and love him. “Those things” are various male lovers. And when Edith is driven home in shock after witnessing Walker’s suicide, her mother is still at work and there are furtive noises and male voices from the Jacuzzi. Before the book ends, Edith will be living in frozen silence with her father while her mother, staying at a motel, looks for a new home.

Walker’s, Joe’s and Edith’s wounds from parental betrayal are woven in a back-and-forth chronological pattern into the terrible events of the present. The scene of a school assembly to discuss the suicide comes before the suicide scene itself. It makes a rich, ironic and terribly moving palimpsest. There are the teachers, sitting frozen on the platform. There is the principal, who has adopted the newly fashionable disciplinary approach--he expels relentlessly--and wears an aloha shirt to show he’s a nice guy. “When people wear the right clothes they think they’ll fit in even if the clothes don’t fit,” Edith reflects.

She will walk out when the principal unctuously refers to the “tragedy.” Tragedy, her writing teacher had said, requires choice. One of the glowing things in this book is the young people’s need to believe what they are taught. The principal’s word is a verbal betrayal of both the much-betrayed Walker and the values his teachers expound.

McNally luminously portrays his adolescents trying to move from day to day on their surfaces and to survive their depths. There is passage after passage of stunning exactness. Spencer, hit by the car, is knocked into a front yard, his head inches from the nozzle of the watering system. When the ambulance arrives, the car’s driver stands in shock beside him, wearing one shoe. He had put his other shoe on the nozzle to keep the unconscious boy from getting wet.

There is an unforgettable scene in which Joe, Edith and Howie visit Walker in the sanitarium. McNally renders the patient’s brilliant exuberance with sudden trailings-off and repetitions. A similar breakdown comes when Walker’s bereaved father cordially receives a visitor and then goes briefly, modestly berserk.

The portraits of Jenna and Ray are skillful in many ways but considerably less effective. Children of the ‘60s, they represent too recognizably complementary types. She is a former radical and free spirit. He is working-class, an Army veteran and a conservative. Both are boundlessly good people who try to help their anguished adolescent charges and to work out their relationship with each other. Both are too damaged--he is alcoholic, she is spacey--to succeed.

When McNally is showing his adolescents in action, he is, until near the end, almost flawless. Less successful are the poetically voiced but fuzzy passages at the start of each chapter which attempt to give a higher shape to the details that follow. At this stage anyway, the author speaks more beautifully through his characters than directly. Which is no small praise, in fact.

The bloody ending--a second suicide and attempted murder--seems misjudged. It is logically plausible as an expression of the continuing chain of damage but it is dramatically overloaded. McNally is a master of character and scene, but his sense of plot is a little forced. Joe, Edith, Walker and Howie are thoroughbreds on a racecourse that distorts their gait.