Strange (but helpful) things started happening around the office. First, it was the random back copy of the magazine on my desk, featuring the article on the building's history; then it was finding fresh mugs of tea on my desk (made just the way I like, which none of my colleagues could ever get right); then some of the others remarked that the office seemed more organised than usual – we're an untidy bunch, and it was strange to be able to find things suddenly where they were supposed to be.

I thought it might be Harriet, let out when I left the door unlocked on Friday. I went down to see Brian in the basement and found I was perfectly warm enough in just my thin roll-neck and shift dress, because the place was suddenly not much colder than the rest of the overheated office. No marked chilliness.

Brian was actually in shirt-sleeves. "Hello, Georgia. Behaving yourself?"

I felt a guilty twinge, but smiled at him. "Haven't been caught yet, Brian." The usual pleasantries out of the way, I asked casually, "Did it take you long to sort out Friday's carnage? I'm so sorry about leaving the door open..."

"Not your fault, always been a wicked draught down here. As it happens, leaving the door open might've solved the airflow problem. God knows how. But something's worked."

I wandered casually over to the filing cabinets that had been clattering their drawers so spookily on Friday night. "I bet these drift open a lot on these uneven floors?"

Brian shrugged. "Not really. They're buggers to open, usually."

"Hmm." I stood in the spot where I'd felt the deep, disturbing chill just before the box files had flown off the shelves. "Lightbulbs ever go out on you?"

"Sometimes. Dodgy wiring."

I turned round slowly, and felt nothing of the strangeness I'd felt on Friday. "Are there any photos of the cellars?"

Brian almost danced out of his chair, bless him. His two favourite topics: photos and the basement. There was a half-drawerfull in one of the wooden boxes which had done for my knee on Friday.

*

Brian and I spent a happy half-hour looking at old photos. It was genuinely interesting, there were some beauties – just nothing of the sort I was looking for. Eventually, I cracked. "Brian, are there any dodgy photos? You know, ones that weren't quite right?"

He pointed to a box behind him. "Try in there. It's where I put all the crap."

I set to, rifling through the box he'd indicated. "Just out of interest, Brian? You spend a lot of time down here. You've never felt anything...strange?"

"Strangest thing is that bloody draught curing itself." He pointed to the shelf where Harriet had pushed all the box files onto the floor. "And the heating seems to work, now."

I found one particular photo which made me stop and shiver. It showed the basement, being prepared to receive our archives, and there was a light smudge to the left-hand side. To me, it looked like the faint image of a young girl in a long dress and apron. I held it up. "This is a good one."

He shrugged. "Shame about that smudge."

"You don't see a young girl?" It was quite clear to me, even if faint.

"No. Always keep your lens clean, Georgia." He glanced at me. "And get your eyes tested, maybe."

"Can I hang on to this?"

"If you want." He closed the drawer. "What's up, Georgia?"

I mentally ran through my options for being honest without sounding bonkers. "I was reading an old article about this place when it used to be a house. Apparently, a kitchen maid went missing in 1892, never solved." I waved around the basement. "No offence, Brian, but this place really creeped me out on Friday. Was just wondering, you know...? There might be a feature article in it, if nothing else."

He took his glasses off, polishing them on his shirt. "It can be a bit creepy down here, but I've got used to it." He leaned himself against one of the haunted filing cabinets and glanced around, then clicked his fingers at me. "Give that photo here again."

I did. He looked at it carefully, both with and without his glasses on. "It does look a bit like an old-fashioned maid, I suppose. Now you've seeded the idea." Then he grunted, and handed it back. "Let me know if you find anything out."

*

I went to Central Library, and smiled sweetly at the middle-aged guy in the archives section. He greeted me warily, remembering me as the clueless girl from the weekend who was chasing medieval nuns.

"Hi. Kitchen maids, today. Specifically, one who disappears in September 1892. Can we find out who she is? Was, I mean."

He smiled, with a slight twinkle in his eye. "At last, an odd enquiry which we might be able to answer. Name or address?"

"Great. Well, I'm Georgia Spencer, and I live..."

"Of the girl who disappeared."

"Oh, yes. Sorry. Harriet. Haven't got a surname." I gave him the address of our offices, though. He pointed to a computer desk. "Have a seat, we'll start with the census for 1891. She should be on there."

*

Indeed she was: Harriet Black, aged 12, domestic servant; place of birth unknown. The archivist wagged his pencil. "That's interesting. Let's see where she is in 1881."

Turned out she was in the workhouse: her status was 'foundling'. The archivist looked genuinely intrigued. "Mystery girl. Dumped on the doorstep by a poor single mother, perhaps." He tapped his pencil on his teeth. "And she disappeared? Might be in the papers."

We moved to a microfilm reader, and he threaded on the local paper for 1892. There was a small article on an inside page:

"Tragic accident

An inquest was opened yesterday on Harriet Black, 13, kitchen maid, whose body was found locked in the cellar of the house where she was employed. The family and most of the servants were away in the country. Testimony from the parlour maid, the only other employee in the house at the time, indicated that deceased had evidently become trapped in the cellars by accident and was found by witness when it was tragically too late. Deceased, a foundling of n***o heritage, had worked for the family for five years, having been left as a baby at the door of the workhouse. A verdict of accidental death was returned."

I pointed to the n-word. "Shocking to see that in print."

"Different times. Might account for her surname, though – the workhouse may well not have known who she was." The archivist stood up. "Poor girl. Talk about disadvantaged. And dead barely into her teens."

"Mmmm." I sat back and digested Harriet's brief and unhappy life, then made to leave.

The archivist got me a copy of the article and of the two census returns. "See you again?"

"I'm afraid you probably will, sorry. Just now, I should get back to the office."

*

"Poor girl," said Brian when I showed him. He glanced around and frowned. "Must've been locked in here for a while."

We both shivered at the same time, and the cellars went very chilly for a moment. There was a rustling from the far end. Then the lightbulb fizzed again, making us both jump. "Harriet?" I put a hand on Brian's arm. "Can you hear me? Er...if you can, knock on something. Once for yes, twice for no."

There was a boink on the top of the filing cabinet.

"Bloody hell," said Brian.

"Harriet, have you been making me tea?"

Boink.

"Thanks. Good job." I folded my arms. "Did you really get trapped down here and die?"

Boink.

"Ask her how long she was down here for, Georgia."

"Ask her yourself. But closed questions only. Yes/no." There was some general rustling and the lightbulb fizzled again, then one of the box files at the bottom end dropped off the shelf. "That's your 'bloody draught', by the way, Brian. She's called Harriet." Another file flew off the shelf, and I stepped forward quickly. "Woah, OK, Harriet. Calm down. Something's obviously upset you. Let's just take a second and see if Brian and I can help." I felt slightly silly offering to help the incorporeal and mischievous spirit of a 13-year-old who'd been dead for around 125 years, but the poltergeist disturbances stopped.

"So, how many days were you down here for?"

Boink boink boink boink.

Brian tutted. I got a nasty creeping sensation on my neck as something randomly occurred to me. "Did the parlour maid know?"

Boink.

"It wasn't an accident, was it?"

Boink boink.

"OK. Promise me you'll stop chucking Brian's stuff on the floor, and I promise I'll try and do something to help you find rest. Deal?"

Brian and I held our breath, and after a long pause there was a final, single boink.