The concept is as high as an elephant's eye. Two students at the University of Edinburgh have a post-finals fling on July 15, 1988 - “one really nice night together” which neither wants nor expects, at that point, to lead to anything more permanent (though a clever coda will revise our perception of events).

One Day revisits Emma and Dexter on this day, St Swithin's Day, over the next 20 years, tracing their lives sometimes in parallel but mostly at moments of charged intersection, when what is obvious to us becomes obvious to them, too: that they are happier, funnier, better people when they are together than when they are apart; that they are meant to be together; that they are in love. As it is, the couple remain friends, best friends in some ways, but their paths diverge radically. Working-class northerner Emma moves to London and becomes a waitress in a bad Mexican restaurant before deciding to become a teacher. She moves in with her boyfriend, Ian, a soulsappingly unfunny stand-up comedian.

Meanwhile, confident, quasi-posh Dexter drifts into the media and ends up presenting a terrible post-pub TV show called largin' it. He dates his co-host, Suki, and becomes addicted to sex, drugs and his own pitiful C-list celebrity. Naturally, Emma dislikes Dexter's belligerent telly persona and, in the chapters that follow their momentous falling out, they meet only at the weddings of mutual friends. We know that they have the capacity to redeem each other but as the years tick by it seems increasingly unlikely that this will ever happen.

You're not convinced, are you? You're thinking, “I don't need to read that, I've seen When Harry Met Sally”, or, “That sounds saccharine beyond belief”. Or, if your bent is more literary, you're imagining walking past racks of One Day in Asda or Tesco, glowing with pride because you never read novels like that, “commercial” romantic comedies with cartoons and squiggly writing on the cover. Well, be convinced: One Day is a wonderful, wonderful book: wise, funny, perceptive, compassionate and often unbearably sad. It's also, with its subtly political focus on changing habits and mores, the best British social novel since Jonathan Coe's What a Carve Up!

What lifts it beyond genre isn't just the writing - Nicholls's witty prose has a transparency that brings Nick Hornby to mind: it melts as you read it so that you don't notice all the hard work that it's doing - but the richness of its characterisations and refusal to provide any sort of easy consolation. For, in spite of its comic gloss, One Day is really about loneliness and the casual savagery of fate; the tragic gap between youthful aspiration and the compromises that we end up tolerating. Not for nothing has Nicholls said that it was inspired by Thomas Hardy.

One of the novel's epigraphs is from Days, by Philip Larkin, a Hardy fan - “Where can we live but days?” It's an apposite choice, but the Larkin poem that buzzed around my head in the grief- stricken hours after I finished One Day was The Mower: “We should be careful/ Of each other, we should be kind/ While there is still time.”

One Day by David Nicholls is Book at Bedtime on BBC Radio 4 for two weeks, starting this Monday at 10.45pm.

One Day by David Nicholls
Hodder & Stoughton, £12.99; 448pp Buy the book

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