It rained every day. We ran to churches and art galleries, and in between we sheltered in little cafes drinking double espressos or hot chocolate. I'd dreamed of long, slow walks through the labyrinth of canals, leaning together on bridges to watch the boats go by, sex under thin sheets with the shutters closed against the sunlight. We spent too much money on lunches, which were meant to have been picnics of bread and cheese, or slices of pizza, because it was better to sit inside for a couple of hours with the tourists' three-course menu and a jug of house wine. Nick bought me a leather wallet and a glass thumb ring. I took photographs of him standing damply on the Rialto Bridge. At night we ate in tiny restaurants and went to bed with the sound of rain clattering against the small windows of our room. He flossed his teeth for five minutes every morning and every evening. He snored in his sleep. He loved chocolate and ice cream.

Every so often, the rain momentarily stopped and the sun half appeared through a gauze of clouds. The puddles glistened and the swollen canals rippled in the light, and the stones steamed. It was the most silent, beautiful city I had ever been in, and I found myself wishing, once or twice, that I was here alone, not worrying about our relationship, not having to make an effort. I would have walked and walked along the deserted paths, not speaking, storing everything up. I wouldn't have minded the rain.

They were still there when I got back on Sunday afternoon. Indeed, they seemed more firmly installed than ever, their belongings spreading along shelves, their laundry in the washing machine, toothbrushes in my London Underground mug. In two thick piles on the table were wedding invitations: Saturday, 13 December, at 4 p.m. They were making lists of who to invite, of decisions to make, tasks to be done. There was an air of bustle and excitement about them.

I unpacked and went to see Laura, but a couple of Tony's friends were there so after half an hour or so I came back. I said to Brendan and Kerry that I had a headache. I made myself scrambled eggs and a cup of tea and took them into my room, shutting the door behind me. I sat in bed, hearing the television next door, the phone ringing and being answered, water running, laughter, the springs on the sofa bed creaking. I poked at my scrambled eggs until they were cold and unappetizing, and stared at my bookshelves and the piles of paper on my desk. Was I imagining it, or did it all look a bit different, as if someone had been tampering with things? I turned off my light and lay in the dark. Brendan laughed very loudly, as if he wanted to be heard. As if he wanted me to hear him.

The next morning, though, they left early to go to the house they were buying. They said they wanted to measure up for curtains and bookshelves, before Kerry went to her office at ten. I decided to arrive at work later than usual, so that I could spend some time alone in my flat.

Later on, I went over and over it in my mind, everything I did in that lovely, quiet, empty hour before leaving. I tidied the kitchen-living room, pushing the duvet and sheets into the tall corner cupboard, folding up the sofa bed, cramming scattered garments into bags, washing plates and glasses from the night before. I opened the windows wide to air the room and rid it of its unfamiliar smell, swept the tiles, vacuumed the carpet. Then I had a long bath and washed my hair. I pulled the plug and cleaned the bath out before sitting down to breakfast in my dressing gown, a towel wrapped like a turban round my head. I ate the remains of the muesli with yoghurt; a big cup of coffee. I even heated the milk for the coffee. Then I got dressed, cleaned my teeth, picked up my overalls and left, locking the door behind me. I know I did all of that. I clearly remember.

I was still working on the big house in Hampstead. Bill dropped in at lunchtime and took me out for a salad. I finished at half past five, cleaned my brushes and drove home. I wasn't seeing Nick that evening, and Kerry had said something about going to a movie, so I thought maybe I would be able to spend time on my own, which I was craving. I could get a takeaway and listen to music, perhaps. Read a book. Mooch.

It was nearly six-thirty when I pulled up outside my flat. There were no lights on, and the curtains were still open. My heart lifted. I ran up the stairs and even as I pushed the door open I heard it. The sound of dripping, tinkling. A tap running. Except it wasn't the same sound as a tap running; it was bigger, more complicated. Then I stepped inside.

There was water everywhere. The kitchen floor was an inch deep in it and the carpet was sodden when I stepped on it. There was water pouring from beneath the bathroom door. I opened it and stepped into the flood; the remnants of the book I'd been reading in the bath that morning floated by the toilet bowl, along with a mushy roll of toilet paper. There was a steady waterfall cascading over the rim of the tub. The hot tap was half-on. I waded across the room and turned off the tap, then plunged my arm, still in its jacket sleeve, into the water to find the plug. I felt ill and sick and consumed with anguish, and then I thought about the flat below and I felt worse. I found a dustpan and started sloshing water off the floor, into the emptying bath.

It took forty-five minutes to get the worst of the water off the bathroom floor. I laid newspapers everywhere to soak up the rest and started on the kitchen. Then the bell rang.

He was yelling before I'd even got the door open. He sploshed across the carpet, still shouting at me. His face was quite purple. I thought he might have a heart attack or a stroke, or he might just die from his head exploding.

'I'm so sorry,' I kept saying. I couldn't even remember his name. 'So sorry. I don't know how…'

'You'll sort this out, do you hear? Every last thing.'

'Of course. If you give me the details of your in -'

At that moment Brendan and Kerry appeared, arms wrapped round each other, faces glowing from the night air.

'What on earth…?' began Kerry.

'You may well ask.' I whirled on Brendan, 'Look at what you've fucking gone and done. You stay here, you clean out my fridge, you drink my coffee and my wine, you take up every inch of space so I can't move without bumping into you. You have bloody baths in the middle of the day and then…' I was spluttering with rage. 'Then you go and leave the plug in and the water running. Look! Look!'

'And that's nothing compared to downstairs,' said my neighbour grimly.

'Miranda,' said Kerry, 'I'm sure…'

'Whoa!' said Brendan, holding up his hands. 'Calm down, Mirrie.'

'Miranda,' I said. 'Miranda. There's no such name as "Mirrie".'

'Don't get all hysterical.'

'I'm not hysterical. I'm angry.'

'I haven't been here today.'

'What?'

'I haven't been here.'

'You must have been.'

'No. Now sit down, why don't you, and I'll make us all some tea. Or maybe a drink would be better.' He turned to my neighbour. 'What about for you, Mr, er…?'

'Lockley. Ken.'

'Ken. Whisky? I think we've got whisky.'

'All right, then,' he said grudgingly.

'Good.'

He pulled the whisky bottle out of the cupboard, and four tumblers.

'You must have been here,' I said to his back. 'You must.'

'I went to look at the house with Kerry, then I went shopping. Then I met Kerry for lunch.' Kerry nodded. She still looked shaken by my outburst. 'Then I went to Derek and Marcia's to see Troy.' He put his hand on my shoulder. 'No midday baths, Mirrie.'

'But…'

'Did you have a bath before you left, maybe?'

'There's no way I left the plug in and the tap running. I don't do things like that.'

'It's so easy to do. We've all done something like that at one time or another.' He turned to Ken. 'Haven't we, eh? I'm sure Miranda will make sure everything's dealt with. And she's in the building and decorating trade, so maybe she can help you with the painting and stuff. Mmm?'