Shock Waves From a Little Boy Lost : FICTION : THE DEEP END OF THE OCEAN.<i> By Jacquelyn Mitchard (Viking: $23.95, 434 pp.)</i>

For many parents, the worst tragedy imaginable would be the death of a child. But in Jacquelyn Mitchard’s fine first novel, “The Deep End of the Ocean,” Beth Cappadora discovers something worse: that a child can simply vanish without a trace, that not knowing where he is or who has him can be more devastating than knowing he’s dead.

The year is 1985. The occasion is Beth’s 15th high school reunion in Chicago. She arrives from Madison with her children, Vincent, 7, Ben, 3, and Kerry, 6 months. Earlier that morning, cranky at having to take along all three kids while her husband stays home to work, she mutters, “I only like the baby.” Hours later, her words come back to haunt her. Because by then, in a hotel lobby jostling with alumni, Ben has disappeared. How? She only turned away from him and Vincent for a moment. But years afterward, looking back, she knows that “everything that matters in life is decided irrevocably in seconds.” And the guilt that follows is unendurable.

As a parent myself, I tend to avoid the pain of reading novels about kidnapped, abused or dying children. Once in a while, though, a book like Mitchard’s rises above the obvious horrors of its subject to tell a bigger story about human connection and emotional survival. “The Deep End of the Ocean” is devastating, yes, but so well observed and perceptive it’s hard to shy away from. It’s also masterfully paced--beginning with a prologue set a decade after Ben’s kidnapping and winding back to the event itself--and the hours, days and years that follow.

At first, the search for Ben starts almost casually, with Beth’s school friend Ellen leaping onto a luggage cart and shouting to classmates, “We all need to look for Beth’s little boy. . . . He’s 3, his name is Ben, and he was here a minute ago.” Gradually, the mood changes. Police arrive, alums are questioned, the hotel is searched. Full of helpless panic, Beth drinks the vodka people hand her and tries to stay strong and upright--for Ben. “If she lay down,” she reasons, “if she rested, wouldn’t Ben feel her relaxing, think she had decided to suspend her scramble toward him. . . ? Would he relax then, turn in sorrow toward a bad fate, because his mama had let him down?”

As hours slide into days, Beth clings to this paralyzed vigilance. She neglects her remaining children and her grief-stricken husband. She stops working, eating and communicating with anyone but Candy, the police detective who heads the investigation. The search leads nowhere. A media blitz inspires only a slew of false confessors. Morgues turn up the bodies of other children--smaller than Ben, older than Ben. Beth visits a psychic who sees Ben in a box.

In the awful limbo of uncertainty, Beth’s marriage begins to crumble. Her husband, Pat, who wants to move on, accept their loss and have another child, calls Beth a martyr and admits he blames Ben’s disappearance on her sloppy mothering. She retaliates with more indifference.

Vincent, meanwhile, nearly as lost as his brother is, suffers from nightmares. His parents, especially Beth, have no time for him. (“He could pop out one of his eyeballs right now and his mother would say, ‘Vincent, take that outside.’ ”) Worst of all, he carries his own guilt over his brother. The day of the kidnapping, tired of baby-sitting as his mom schmoozed with her friends, he’d told Ben to get lost.

As events and emotions ripple out from the Cappadoras’ tragedy, Mitchard, a columnist for the Milwaukee Journal, poses questions that occur to anyone who has studied faces on a milk carton. How does the child of responsible parents drop out of sight? What happens to a family if the child never is found? Or what if, years later, he is found--unharmed but a stranger?

The pain of Mitchard’s story is somewhat leavened by her moving evocation of everyday life--and of people struggling mightily to cope when that life goes awry. Minor saints come forward: Grandpa Angelo, cuddling the neglected Vincent crooning, “My little love. My best boy.” Candy Bliss, the detective with the stripper’s name pulling the dazed Beth to her feet.

Beth herself, the book’s most complicated character, is also its least admirable--shattered rather than strengthened by her loss, almost catatonic in the face of others’ needs. In other words, she’s a fully flawed and believable human being. Let’s hope when the movie comes--and it’s already in the works--Beth won’t be sanitized for the screen. This isn’t a story of stirring heroics but of lurking darkness, and of one family’s slow stumble back into light.