Emma signed the delivery man’s digital pad and watched as the box was manhandled into the hall by his burly associate. As soon as the door had closed behind them Emma ripped off the tape and flung open the cardboard flaps. Inside lay a shiny metal figure, about the size of a small adult. A single printed page lay on what would have been its chest if it were human. It had an oval head with two slits about where you’d expect to see eyes on the front, and small disc-like protuberances which looked a little like ears on the sides.

Emma glanced at the instructions, located the tiny button behind the left ear, pressed it and stood back. She heard a series of clicks, then the eye slots flashed open and a steady green light shone out. Emma gave a tiny jump and squeak and clapped her hands. Beside her, 12 year old Lucy rolled her eyes. She’d seen too many of her mother’s new gadgets to be impressed by a robot. Robots were so last century. With her best withering glance she slouched off to her room.

Emma didn’t notice. She was reading the Quick Start sheet. ‘FDI Robot,’ she said loudly. The head swivelled towards her. ‘Hello,’ it said. The voice was pleasant, mellifluous. ‘How may I serve you?’

Emma looked at the sheet again. ‘Get up and tidy away your packaging.’ The machine’s hidden gears were just audible as it sat up, then tucked its metal leg under itself and rose. Soon it was standing in front of Emma, humming with purpose. The cardboard box and all the tape had been compressed into a small square and binned.  

‘I am Family Diet Improvement and General Household Robot 7548-897-0-12 (AA). Do I have the honour of addressing Emma Stewart-Leslie, 40 years of age, address 89 Petunia Way, Milton Keynes?’

Emma was thrilled. This was better than she had imagined, apart from the age bit. ‘You do. I mean you are. Whatever. I’m Emma.’

‘Pleased to meet you Emma.' It reached out a smooth plastic hand. It felt oddly warm, and made Emma's flesh crawl a little as she shook it. 'I am here to help you and your family, Harry Stewart-Leslie, aged 42 years, and Lucy Stewart-Leslie, aged 12 years, achieve the weight target you desire. But first you must read this.’ It plucked a small, thick book from a slot in its hip. Emma flicked through it. Too long. She was in a hurry.

‘Excellent,’ she said. She put the book on the hall table. ‘Simply, what I want is for all of us to lose as much weight as possible, as quickly as possible. We’re going on a cruise in six weeks’ time and I want us to look thin.’

‘I will start right away,’ said the shiny, slim machine. ‘Please direct me to your kitchen.’

Emma watched as it inspected the contents of the family fridge. It started taking packages out and collecting them in a bin bag. She didn’t notice it watching her as she plucked a packet of cheese and onion crisps from the multipack in the larder. After she had settled in front of the tv to watch her favourite hospital soap the robot opened the larder and dumped the crisps into the bin bag. It studied the rest of the contents of the larder and got busy.

That evening, Harry stood in his empty, clean kitchen and stared at the robot, which was preparing a salad. ‘And just what is that?’ he asked Emma.

‘It’s our new diet control robot. I call it Lady Di.'

‘How much did it cost?’

‘We can afford it. What we can’t afford is to board the Caribbean Princess looking fat.’

‘I don’t like it,’ said Lucy from the doorway. ‘It’s creepy.’

Emma looked at her daughter. At 12 she should have lost her puppy fat. ‘It will be good for all of us,’ she said firmly. ‘Every year we say we’re going to lose weight in time for our summer hols. This year we’re going to follow through.’

Dinner that night was not a happy affair. Emma tried to put a brave face on it, but even she picked at her kale and cabbage salad and couldn’t quite bring herself to finish it. The next few days were even tougher. The fridge and pantry were empty. There were no crisps in the pantry and no digestives in the biscuit tin. In fact, there was no biscuit tin any more. And Di was always on guard. It never slept. When it wasn’t preparing low-fat meals which they all hated and which utterly failed to satisfy their hunger it was stopping them getting out to buy food. After Lucy succeeded in having a pizza delivered the internet was cut off.

After the first week, however, Emma was pleased. Her family were trim, even Lucy. Sure, they complained, but they all had to suffer to look good. Emma missed her crisps and her wine. Harry missed his biscuits, and his daily bottle of beer. Lucy complained about not being allowed out of the house or to contact anyone outside the house, and began to spend a lot of time in her bedroom. She hated her parents even more than most 12 year-olds. But they were all getting thin.

Then things changed. One Sunday, three weeks after Di’s arrival, Lucy didn’t emerge from her room all day. In the evening, Harry tried to lift the emaciated child from her bed and fell over, too weak to carry her. ‘Dad, we need to get food. That thing’s starving us.’ Emma, leaning on the doorframe, heard Harry’s muttered agreement.

‘You’re both weak,’ she croaked. ‘You’ll thank me in the end.’ They both watched as she slumped forward into the room and lay face down on the carpet. Behind her, Di slowly closed the door. Lucy, the last one left conscious, heard the key turning in the lock.