Harriet Walsh stood at the door to her aunt's apartment, steeling herself. It was only two days since the funeral, and the modest flat held so many memories she was tempted to walk away.

Unfortunately, she had nowhere else to live.

Harriet shook herself, opened the door and carried her shopping to the kitchen. She emptied the bag onto the old wooden table, placing a small carton of milk and a loaf of bread side by side. Then she fetched a mug and a plate from the cupboard above the sink. She tried to ignore the familiar mark on the kitchen counter, from the time she'd baked her first cake, years ago, and the hot tin had left an outline. Every room held similar memories, the entire flat like a time capsule from her childhood.

Harriet's stomach growled as she took a seat, and she fought down the temptation to rip open the milk carton and drain the contents, following it down with huge mouthfuls of bread. Instead, she poured half the milk into the mug, took out two slices of bread, and laid them on the plate.

She looked around the small kitchen as she ate her dinner. It was evening, and almost dark outside, but there was a deeper gloom in the apartment which the single light above her head would never dispel. Not even if she had the electricity to turn that light on. It seemed Auntie's cheerful spirit was the only thing that had kept the modest apartment alive. Now, with Auntie gone, the place was smaller somehow, and thoroughly depressing.

Harriet took another bite of bread. Across the table she could see two envelopes, and the sight didn't lift her spirits. One had the Real Estate agency's logo, and she knew it would be about the rent. The other was unmarked, which probably meant junk mail. She wasn't in a hurry to open either of them.

Still, she thought sarcastically, there's nothing like a good read while enjoying your evening meal.

Dear Ms Harriet Walsh,

I'm delighted to inform you that your name has been selected —

Harriet rolled her eyes and discarded the first letter. Unfortunately, the second was even worse:

Dear Ms Harriet Walsh,

It has come to our attention that your rent is now in arrears to the tune of three months. While we knew and respected your aunt, and we understand this is a tragic time for you, we must also remind you that previous demands for payment have not been met.

Therefore it is my duty to inform you must vacate your apartment within three days. Please ensure the fixtures and fittings are in original condition, and that all furniture and belongings are removed.

Harriet set the letter aside, her insides cold. Of course she had no money … she'd spent most of this past year caring for her aunt! She didn't regret a second of it, not after everything Auntie had done for her, but the effort had left Harriet broke and worn out.

And now, it seemed, homeless.

Well, at least she had a couple of days … maybe she could find a casual job and beg for an advance. There were plenty of positions caring for the aged, what with Dismolle being a well-known retirement planet, but she couldn't face that again. Not so soon after Auntie.

Harriet eyed her supplies. The other half of the milk would do for breakfast, and the bread might last a couple of days before it went mouldy. No problem, then. Plenty of time.

As she ate, Harriet picked up the first letter and scanned it idly. She'd received two in the same vein over the past couple of weeks, and had recycled them without bothering to read the things. Actually, she'd used them to mop up spills and then recycled them. Waste not want not, as Auntie always said. Harriet's lips thinned at that.

As she skimmed the flowery sentences, Harriet realised she'd been mistaken. The letter wasn't a scam or a lottery, and it wasn't asking for money. No, it seemed to be offering her a job … and it wasn't caring for the elderly. Taking a deep breath, she read it more carefully.

Dear MS HARRIET WALSH,

I'm delighted to inform you that your name has been selected from a vast pool of potential candidates!

As you are no doubt aware, Planet Dismolle is a peaceful haven, with no major crime or civil disturbances. It is now ten years since the last remaining Peace Force officers were re-assigned, leaving a token office and a skeleton staff to handle minor misdemeanours and infringements.

I'm now in the happy position of being able to add an extra officer to my team, on the understanding that I personally train said officer. To that end I have scoured population records to locate the most suitable candidate, and only one person met every criteria. That person, MS WALSH, is you.

Therefore, please report to the address herewith enclosed in order to begin your training.

Dismolle Peace Force

Capital City

Harriet frowned. She was no detective, not yet at least, but something was off. For a start, when she turned the letter over she discovered it was printed on the back of a fast food flyer. She paused at the sight of the meals, eyeing the succulent burgers and fries, the chocolate shakes, the ice cream sundaes. Her bread and milk suddenly tasted bland and sterile.

Hurriedly, she flipped the letter over and continued her inspection. There was no official logo, or letterhead, or whatever. The signature looked like it had been written with a sharpened fence post dipped in ink, running halfway across the page in huge jagged strokes. Then there was her name, which appeared in a different typeface to the rest of the letter, almost as though it had been stamped in afterwards. Finally, the letter was undated and there was no time specified for her appointment.

Then there was the whole idea of the thing. Who the hell thought she'd be suitable for the Peace Force? She'd done okay at school, but had never been really good at anything, except maybe sport. Cross-country running was the only thing she'd enjoyed, and she'd given that up when the workload became too intense. She disliked puzzles, had never even read a detective mystery, and hated wearing uniforms.

In fact, the only positive she could see in joining the Peace Force was a regular pay cheque.

"Good enough," said Harriet, and having made the decision she took three more slices of bread, poured the last of the milk into her cup, and set about demolishing her feast.

Public transport was free of charge on Dismolle, a fact for which Harriet was truly grateful. As the driverless cab took her into the city, she gazed out of the window, taking in the neat houses and gardens. There were robots tending the lawns, robots doing minor repairs … even robots taking dogs for their morning walks.

She thought back, trying to remember if she'd seen another living human throughout her trip. She didn't think so but she wasn't certain, and once again she wondered what the Peace Force could possibly have chosen her for. She had all the observation skills of a wooden plank, and as for her memory …

She was convinced the letter was a hoax, but she had to go into the city anyway. If the Peace Force thing was a bust, she'd head over to the job office instead. She could always get a job sweeping paths … or walking dogs. Then she realised what she was thinking, and shook her head. Anything she could do, a robot could do better. Anything except looking after the elderly, who instinctively mistrusted robots, and were in turn mistrusted by them. Nothing scrambled a robot's mind like witnessing a human death, and the frail elderly of Dismolle were sadly all too prone to it.

"So it's one or the other," muttered Harriet. "Peace Force or aged care."

"Too many destinations," said the cab, in an electronic male voice. "Please state a single destination."

"Proceed as before," said Walsh clearly, and she kept quiet for the rest of the journey.

The cab drew up at Dismolle's one and only Peace Force station, and Harriet craned her neck to get a better view as she stepped out. Most buildings on the planet were low and airy, with big windows and good lighting. Whoever built this place hadn't got the memo, because it was a multi-story chunk of concrete that blended with the streetscape like an active volcano. With its slitted windows, thick iron bars and concrete blocks protecting the entrance it looked solid and impregnable, like some kind of war ministry bunker.

It also looked old, and she wondered whether it had always been a Force office … or whether it had once been a weapons silo. Not that the good folk of Dismolle did wars, of course. They barely did heated arguments.

Harriet shrugged and headed for the door. On the way she pulled out the letter, which was crumpled and slightly damp from wiping the kitchen table down. Waste not, want not …

The door was a double glass affair with dark tint. A huge Peace Force badge was etched into the surface, the gold foil peeling and grimy. As Harriet approached the doors, they swept open … but only about three inches. Then, with a nasty grinding noise, they jammed. Frowning, she tried pulling them apart, but withdrew her hands when she realised the doors might close on them. What a terrific first impression that would make, if she had to be rescued from the front doors before she even got to the interview.

Since she couldn't get in, she put her mouth to the gap. "Hello? Is anyone there?"

There was a delay, and then she felt the ground shake underfoot. It was like someone had pounded on the paving with a big, heavy mallet, and as she stood there, head on one side, she felt the floor shake again … and again. The thuds were getting stronger, and Harriet looked up at the building's facade anxiously. If it was maintained as well as the front doors, lumps of concrete might come tumbling down on her head.

Instead, the thudding stopped and a pair of hands appeared between the doors. Harriet noticed they weren't human, and she didn't need her modest deductive skills to work that out. First, they were twice as big as the biggest hands she'd ever seen. And second, they were fashioned from dark blue metal.

Through the tinted glass, Harriet saw the vague outline of a hulking great robot. Through the gap between the doors she saw its face, which had a curved panel where the eyes should have been, and a very human mouth right where … well, right where a mouth ought to be. She didn't have time to see any more, because at that moment the robot flexed its massive arms … and the doors shattered into a million fragments.

"Oh dear," said the robot, in a slow baritone. There was a female element to the voice, and Harriet wondered if the designers had been going for the speech pattern of an elderly librarian. Something less likely to scare small children and the elderly. Something to lessen the impact of the huge body, massive hands and thudding footsteps.

If so, they'd failed.

The robot was about two metres tall, nearly one and half metres wide, and had a pair of feet on it like upside-down trash cans. Its arms and legs would have put a spaceship's landing gear to shame, and the oversized chin could have doubled as a snow plough … or an icebreaker.

Now Harriet understood the shaking floor … and possibly, why the building was built from sturdy concrete blocks. Then she forgot the building and started smoothing out the letter, because if this giant robot decided she was a criminal turning herself in … well, she didn't want to get arrested by it, that was for sure. "I'm here for the trainee thing," she said quickly, holding out the letter.

The robot ignored her. It was still inspecting the remains of the door, shaking its massive head and saying 'oh dear, oh dear,' over and over again. It tried to sweep the glass aside with its foot, but instead of clearing the entrance, it ended up grinding the fragments to a kind of glittery powder. "Oh dear," it said again, then tried with the other foot in case that worked any better.

"Hello?" said Harriet. Robots were supposed to multi-task better than humans, but this particular model could barely single-task. "I'm Harriet Walsh."

That got a reaction. The huge robot stopped grinding glass fragments with its feet and looked at her. Red lines on its eye panel swept back and forth, up and down, and Harriet wondered if they were radioactive beams, x-rays, or just cool-looking lasers.

"You came," said the robot simply.

"I got your letter," said Harriet, holding it out again.

"Many letters have been sent out over the years," said the robot. "None elicited a response."

"Well, this sort of thing is usually a scam."

"A … scam?" The robot fumbled with its thick fingers, before eventually taking hold of the letter.

"It's, er …" Harriet realised it might take a while to explain human trickery to what was apparently a very simple robot. "Fake letters," she said at last. "False."

"Oh, the letters were all genuine." The robot held the sheet of paper to its 'eyes', and scanned it. "This letter is also genuine."

"Well it doesn't look it. I mean, it's printed on the back of a fast food leaflet."

"I am programmed to conserve and recycle precious resources."

"Well it looks like a joke. All the other people probably threw theirs away."

"Other people?"

"You said there were many letters over the years. The other people you sent them to must have—"

"There were no other people, Harriet Walsh," said the robot gravely. "For the past six years I have been sending these letters to you alone. And now, finally, you are here."