THE CHILD IN TIME By Ian McEwan Houghton Mifflin. 263 pp. $16.95

Two and a half years ago Stephen Lewis and his wife Julie suffered an unspeakable loss: Their 3-year-old daughter and only child Kate was stolen by a stranger while she and her father were at a London supermarket. She has yet to be found, and in the long months since her disappearance her parents have been driven apart by guilt, recrimination and sheer inexpressible grief.

"There was no room for anger, no openings. They moved like figures in a quagmire, with no strength for confrontation. Suddenly their sorrows were separate, insular, incommunicable. They went their different ways, he with his lists and daily trudging, she in her armchair, lost to deep, private grief. Now there was no mutual consolation, no touching, no love. Their old intimacy, their habitual assumption that they were on the same side, was dead. They remained huddled over their separate losses, and unspoken resentments began to grow."

So now they have gone their own ways: Julie to a modest country place, where she neglects her musical career, and Stephen to their flat in the city, where he drinks too much and abandons the writing of books for children, at which he had made a great success. His only real escape from his grief lies in his friendship with his former editor Charles Darke and in the occasional meetings of the Subcommittee on Reading and Writing of the Official Commission on Child Care, to which Darke has arranged his appointment.

Stephen attends the meetings, but he scarcely pays attention to the proceedings. Instead he wanders in a reverie through the past, his own and his parents', musing about the nature of childhood and the mysteries of time. For him not merely does Kate live on in "the timelessness of memory," but time itself is an eerie, yet oddly comforting, intermingling of past and present; the more he muses, the more he finds himself reflecting upon the nature of childhood and upon his friend Charles' observation that children "really understand that it won't last, that sooner or later they're finished, done for, that their childhood is not forever."

Oddly enough, though, Charles himself cannot accept this. He has had an enormously successful career, first in publishing and then in politics, where he had become a favored prote'ge' of the prime minister; "Darke had no political convictions, only managerial skill and great ambition." Yet quite suddenly he has resigned his government position and retreated to the countryside with his wife, a physicist a dozen years his senior. Visiting Charles, Stephen discovers that this friend who once had seemed the embodiment of maturity has regressed into a tree house: "Once a businessman and politician, now he was a successful prepubescent."

Unfortunately it is at this point that "The Child in Time" begins to get rather out of hand. What Ian McEwan clearly has in mind is to document the aforementioned timelessness of childhood, to show how the child is never fully dead within us, but the means he has chosen simply does not mesh comfortably with the real business of the novel, which obviously is the story of Stephen and Julie and Kate.

"The Child in Time" is one of those novels in which theme and story never quite connect; rather than weave his themes about time and childhood into the tale, McEwan is reduced to exploring them in the most overt and obtrusive way. Not merely are we given Charles' quite pointed lapse into boyishness, but substantial passages are devoted to debates within the subcommittee over various issues pertaining to the care and raising of children; though often eloquent, these passages are unfailingly didactic, and have the air of having been attached to the novel rather than growing naturally from it.

More's the pity, because when he sticks to the main business at hand McEwan is utterly in control. Several scenes involving Stephen and Julie, both together and apart, are quite beautiful -- in particular, an abortive attempt at reconciliation -- and the account of Kate's abduction is heartbreaking; there is also a wonderful incident in which Stephen becomes involved in an auto accident. McEwan is a marvelously gifted writer, and his prose positively sings; it's too bad that he didn't have enough confidence in his story to let it speak for itself.