I'm sitting on the Tube when, somewhere between Kennington and Oval, there's a mashing of wheels, a gentle hiss and the lights go out.

Silence. Tense mutterings and a general leaking of breath. In the half- hearted darkness, people emerge as vague outlines, the details filled in as my eyes adjust.

I reluctantly put down the novel I'm reading. It's The Insult, by Rupert Thomson, one of those rare, perfect, all-consuming books where your life takes on a new colour and texture for the days you are in its thrall. I wait. The lights flick on again - I lift the book - then off. Then on, long enough to begin reading again. I've now read the same sentence - "He had shoulders like the slopes of a volcano and diamond studs in both his ears" - three times. I decide to look at the people opposite me instead.

A woman stares straight ahead, shoulders hunched, holding what has to be a funeral wreath. I realise I noticed her some time ago but was so deep in the book that some part of my brain rejected the image as unlikely. Now I look again.

She's about 60, black jumper, black skirt, torn leather jacket, shabby high-heels. Her straggly hair is grey and uncombed, earlobes distended by the weight of her long, gold ear-rings. Her dazed half-smile reminds me of the women who try to press twists of dried lavender in silver foil into your hands on Oxford Street. The wreath is pink and yellow and lilac - the passive, sickly, paid-for colours of death.

I slide my eyes along. A man, under 30, black leather jacket, black jeans, black cowboy boots. His strawberry blond hair curls down almost to his chest - a great, thick, Aphrodite tangle. There's a touch of womanish fat around his jaw, above the silver buckle of his jeans belt. His eyes are closed, his face blank.

The train shudders as if it might get going, then slides back into silence.

To the left of this man, there's a boy. His black knitted cap is pulled down low on his head. Black jeans jacket, black DMs. His nails are bitten down to the quick, he twitches now and then and has grey, glue-sniffer's skin. I look at him, he looks at me.

The sound system crackles out a message. Everyone strains to listen. Totally indecipherable, it finishes. Silence.

Suddenly, amazingly (because I thought they weren't together), the woman passes the wreath across the long-haired man's lap to the boy. The boy takes it, fingers it, sniffs it without a word, holds it in his lap. His face fills up with pain and for a moment I think he's going to cry.

I sit up. Is she his mother, then? Their mother? Are they a family travelling on the Northern Line to the father's grave? Or even his funeral?

I am considering this new and depressing possibility when the woman smiles and touches the long-haired man on the thigh. Before I can come to terms with this development, he bends his girlish face to hers and kisses her - a long, busy kiss on the lips. While they do this, the boy stares down at the wreath.

Then the lights flick back on and everybody blinks. The train grinds back into motion. Only three or four minutes have elapsed. The lovers pull apart and I plunge gratefully back into my book.

Later that evening I go for a swim. The pool is quiet, cold, shadowy, dim-lit. Just a few serene, white-haired people doing a stiff-necked breaststroke. At the other end, a man has begun to hose the tiles. I wade into the water, hold my fingertips up and suck in my stomach just for a second before going in. Then I lie back, kick, count at the dusty panes of glass in the ceiling. Someone has left a pair of flip-flops and a towel on the side. A woman with defined, arched eyebrows wears a rubber hat with bright- coloured, rubber flower petals all over it. I think my mother had one once.

Afterwards, the changing-room smells of chlorine and hot water and Vosene shampoo. Women who have come straight from the office change back into their striped shirts and tailored skirts and comb their short, wavy hair. Someone called Caroline is talking about her friend's catering business. A plumpish woman sits naked except for a grubby towel, deep in an Argos catalogue. On a bench next to me, an elderly lady sits alone, dry in her swimsuit, staring baffled at her bare feet as if she's forgotten why she's come.

At home, the kids are long asleep and Jonathan is finishing some work - I can hear the faint tap-tap of his computer keys, the occasional long drawn-out sigh. I throw a towel over my pillow, strip off and get straight into bed with damp, chlorine-tinged hair and The Insult. It's about a man who has been shot in the back of the head in a supermarket car park and goes blind. His life seems to be turning into a plot against him, so he runs away from it. In his new life, he sleeps with a girl called Nina. Then his new life turns into a plot against him.

I read, and as I read I think of how, if people can really write like this, then it changes the world for me, alters the balance somehow for the better. Is that how it feels to believe in God?

I think of how really good novels are preposterous, unfathomable things. And how, in two or three days' time, when I am forced to finish this one, my life will be both less and more.