'Well, I wash my hands of it,' says Jemima, still shaking her head. 'What are you going to wear?' Her eyes narrow. 'Where's your outfit?'
'My black dress,' I say innocently. 'And my strappy sandals.' I gesture to the back of the door, where my black dress is hanging up.
Jemima's eyes narrow even further. She would have made a really good SS officer, I often think.
'You're not going to borrow anything of mine.'
'No!' I say indignantly. 'Honestly Jemima, I do have my own clothes, you know.'
'Fine. Well. Have a good time.'
Lissy and I wait until her footsteps have tapped down the corridor and the front door has slammed.
'Right!' I say excitedly, but Lissy lifts a hand.
'Wait.'
We both sit completely still for a couple of minutes. Then we hear the sound of the front door being opened very quietly.
'She's trying to catch us out,' hisses Lissy. 'Hi!' she says, raising her voice. 'Is anyone there?'
'Oh hi,' says Jemima, appearing at the door of the room. 'I forgot my lip-gloss.' Her eyes do a quick sweep of the room.
'I don't think you'll find it in here,' says Lissy innocently.
'No. Well.' Her eyes travel suspiciously round the room again. 'OK. Have a nice evening.'
Again her footsteps tap down the corridor, and again the front door slams.
'Right!' says Lissy. 'Let's go.'
We unpeel the Sellotape from Jemima's door, and Lissy makes a little mark where it was. 'Wait!' she says, as I'm about to push the door open. 'There's another one at the bottom.'
'You should have been a spy,' I say, watching her carefully peel it off.
'OK,' she says, her brow furrowed in concentration. 'There have to be some more booby traps.'
'There's Sellotape on the wardrobe, too,' I say. 'And … Oh my God!' I point up. A glass of water is balanced on top of the wardrobe, ready to drench us if we open the door.
'That cow!' says Lissy as I reach up for it. 'You know, I had to spend all evening fielding calls for her the other night, and she wasn't even grateful.'
She waits until I've put the water down safely, then reaches for the door. 'Ready?'
'Ready.'
Lissy takes a deep breath, then opens the wardrobe door. Immediately, a loud, piercing siren begins to wail. 'Wee-oo wee-oo wee-oo …'
'Shit!' she says, banging the door shut. 'Shit! How did she do that?'
'It's still going!' I say agitatedly. 'Make it stop. Make it stop!'
'I don't know how to! You probably need a special code!'
We're both jabbing frantically at the wardrobe, patting it, searching for an off-switch.
'I can't see a button, or a switch or anything …'
Abruptly the noise stops, and we stare at each other, panting slightly.
'Actually,' says Lissy after a long pause. 'Actually, I think that might have been a car alarm outside.'
'Oh,' I say. 'Oh right. Yes, maybe it was.'
Looking a bit sheepish, Lissy reaches for the door again, and this time it's silent. 'OK,' she says. 'Here goes.'
'Wow,' we breathe as one as she swings the door open.
Jemima's wardrobe is like a treasure chest. It's like a Christmas stocking. It's new, shiny, gorgeous clothes, one after another, all neatly folded and hung on scented hangers, like in a shop. All the shoes in shoe-boxes with Polaroids on the front. All the belts hanging neatly from hooks. All the bags are neatly lined up on a shelf. It's a while since I borrowed anything from Jemima, and every single item seems to have changed since then.
'She must spend about an hour a day keeping this tidy,' I say with a slight sigh, thinking of the jumble of my own wardrobe.
'She does,' says Lissy. 'I've seen her.'
Mind you, Lissy's wardrobe is even worse. It consists of a chair in her room, on which everything is heaped in a great big pile. She says putting stuff away makes her brain ache, and as long as it's clean, what does it matter?
'So!' says Lissy with a grin, and reaches for a white sparkly dress. 'What look would Madam like this evening?'
I don't wear the white sparkly dress. But I do try it on. In fact, we both try on quite a lot of stuff, and then have to put it all back, very carefully. At one point another car alarm goes off outside, and we both jump in terror, then immediately pretend we weren't fazed.
In the end, I go for this amazing new red top of Jemima's with slashed shoulders, over my own black DKNY chiffon trousers (£25 from the Notting Hill Housing Trust shop) and Jemima's silver high heels from Prada. And then, although I wasn't intending to, at the last minute I grab a little black Gucci bag.
'You look amazing!' says Lissy as I do a twirl. 'Completely fab!'
'Do I look too smart?'
'Of course not! Come on, you're going out to dinner with a multimillionaire.'
'Don't say that!' I exclaim, feeling nerves clutch my stomach. I look at my watch. It's almost eight o'clock.
Oh God. Now I really am starting to feel nervous. In the fun of getting ready, I'd almost forgotten what it was all for.