I was wiping the last plate thoughtfully when my phone pinged.

A missed call from Zach.

One of several, I realised with a thud. Urgently, I pressed redial: it went straight to voicemail.

‘I’m just going to…’ I stepped over the log seats, the others lost to Coldplay.

Heading towards the track, I caught a glimmer of electric light through the trees to the east, near the pool.

I hurried towards it, passing the huge oak tree with the frayed old rope hanging from it that the kids had swung from earlier.

Two of the teenagers stood in the shadows on the bank: Zach and Willow, I saw, with some relief – calling to the third.

It took me a moment to realise Cecilia was actually in the water – dancing and twisting in the dark shallows, splashing vigorously as she turned around and around.

‘Come out!’ Willow’s face wasn’t visible in the gloom, but her voice was full of concern. ‘Please, Cecilia.’

‘Mum!’

‘Zach?’ I rushed towards him, almost tripping on a bramble. ‘What’s going on? I don’t think swimming in the dark’s a good idea.’

‘We’re not – it’s only Ceci.’ He met me at the top of the bank, his face daubed with something I couldn’t see. ‘She’s being really weird, Mum.’

I followed his gaze to the girl, waist-deep in the black water, now splashing more manically.

‘She kept saying she felt really hot,’ Zach said. ‘I tried to stop her going in, but it’s like she can’t hear me.’

‘Ceci,’ I called gently. ‘I think you should come out now, OK?’

‘Hey!’ Cecilia spun round again, stumbled and fell back with a great splash.

‘Cecilia!’ I scrambled down the bank, searching the dark water.

She had completely disappeared.

‘Ces!’ Zach cried urgently, beginning to wade in.

Willow rushed down to me as I began to pull my cardigan off.

There was a massive gasp as the girl broke the surface again, coughing and spluttering. I hoped to God she was wearing shoes in there.

‘Ces!’ Zach yelled. ‘Come out, now! You’re freaking me out.’

‘All right!’ She grinned, all teeth in the dark. ‘Keep your hair on!’

‘OK?’ Cecilia took my offered hand as she clambered out.

‘Yeah,’ she murmured, coming to rest, dripping, between Zach and I. Up close, I could see her face was also dabbed with something like warpaint, thick streaks of it on her cheeks, her nose and chin, making her grin seem whiter and madder.

Then she was definitely not grinning anymore, a look of sudden terror flashing across her face.

‘Bit cold now, I’ll bet?’ I bundled my cardigan round her. ‘Let’s go and get you a towel, or a blanket.’

But abject desperation illuminated her features.

‘Oh God,’ she moaned, doubling over. ‘I don’t feel very good.’

‘In what way, Ceci?’

‘I don’t – I feel really sick.’

‘Oh dear.’ Patting the girl’s damp back, I thought queasily of the chicken drumsticks on the barbecue earlier. I’d steered Zach away from them, salmonella being a particular fear of mine. ‘Shall we get you back to—’