GRAHAM SWIFT has an uneasy relationship with plot. His last novel, The Light of Day, was a refracted detective story, a murder mystery in which the malefactor was identified at the very start and the mystery turned out not to be a mystery at all. The novel took place around the edges of this non-plot, and its strange directionlessness gave it a surreal kind of force. Swift's ninth novel is a far more traditional book in many ways the opposite. It has its mystery, and its confessor, and its twists and turns before a big reveal. Perhaps as a result, it's a much slighter work of fiction.

Tomorrow is the imaginary monologue of Paula, a fiftyish mother of twins who lies awake beside her husband on a night in 1995 working out the life-changing story she'd like to tell her adolescent children the next morning. She is an art dealer; her husband, Mike, runs a successful pop-science magazine and publishing business; the novel is the record of their courtship in the 1960s and of how their children were conceived.

But it's a history of silent misdirections and missing links, containing a radical truth about which the kids, now 16, have no idea. "I think you'll want to know everything, " Paula thinks, "the full, complete and intricate story. And you deserve it, as a matter of record."

Long stretches of Tomorrow are very good indeed. Paradoxically, Swift's decision to set this book in a milieu that is probably familiar to a large part of his readership comes off as a brave one, an expression of confidence in the capacity of the known to surprise.

This is a writer of easy subtlety, who specialises in the sidelong illumination of ordinary details and whose writing voice gets its best effects from tiny but striking abstractions: "There was a pause in which I think I could detect, as surely as Mike could with his ear pressed to the receiver, the sound of a man being changed into a grandfather, " says Paula, or she observes of life with the twins that "we've been living in the binary system. . . that strange equilibrium, a family of two couples."

So this is an interesting book in many ways, but it's one with a critical flaw at its centre . . . not an uncommon flaw in first-person fiction, but still surprising in the work of so good a writer.

Fundamentally, the narrator doesn't convince. Paula has had a lot of work done on her: her conversational register, her cliches, her suburban setting, her personal and peculiar delight in language. But identification falls down at the point where she's also required to be a novelist.

The big secret in Tomorrow is the main thing in the novel: nearly everything that's told stems from it. But Paula is persistently and effortfully made to confect it . . . for the reader's benefit, and to extend the skeleton of the plot . . . in a way that would be monstrous if she were really doing it for her children. "I'm your mother, " she says, "and now the truth is going to be uncovered, there should be no little residues of secrecy, " yet every tenth page or so for the first 150 we find her dropping some dark nowread-on hint about the revelation in store. "She's one of the never fully explained mysteries of your life, though believe me, not the only one, " she says, or "You see the way . . . the unfortunate way for you . . .

this is heading?" or "We'll find out soon just what you're made of."

The standard defence of this kind of literary contrivance, besides the obvious one that novels have to get written somehow, is that it mimics the narrator's difficulty of forming and expressing difficult truths.

But that rather collapses here, not only because of the authorial heavy breathing but because of the narrative's other signs of literary coercion: it makes rather a dull thud nowadays, during the monologue of an unfaithful wife in bed in the early hours of the morning, to conclude a chapter with "'Yes, ' I said. 'Oh yes, yes, yes!'" Tomorrow takes a situation that teeters on the banal and tries with some success to make it new.

Here, though, you can see the puppets moving. Despite the manifest virtues of this midnight soliloquy, it's hard not to be relieved when it starts getting light outside.