I spent the next few days in a mood of frustrated impatience, trying to decide what we should do next. We knew that the plans had been stolen by Boulet and sent to the mysterious blackmailer, but with only the number of a box in a small post office, we had no idea of who the blackmailer (and presumably Poirot's murderer) could be. To my annoyance I kept coming back to Japp; I knew if I passed the information we had on to the Inspector he'd be onto the post office in a flash, and possibly lead us to the murderer. But that would cause trouble for Allerton and Boulet, and I'd given my word not to betray them. For that matter there was no guarantee that the PO Box would give us any further aid: the plans had been offered for sale, so the blackmailer had clearly retrieved them and might well not bother returning to the box again. They would have registered their details when they hired the box, but they were unlikely to be truthful – if I were to indulge in blackmail myself I would certainly give a fictitious name and address to register a post office box.

I was unable to sit and think quietly like Poirot did, so I spent much of the time fidgeting in the sitting room, unable to settle, or talking to Miss Lemon in her office. Even though Poirot was no longer around, she still seemed to spend a significant amount of time on her typewriter. "What exactly are you working on, Miss Lemon?" I asked one morning.

"There's still a lot of mail coming in for Mister Poirot, Captain Hastings. I'm sending them out replies informing them he won't be able to take on their cases as a courtesy." She told me.

"Are you-" I began and then stopped. She looked up at me expectantly, so I reluctantly continued "Are you looking for a new position?" I hated to ask the question, but desperately wanted, in fact needed, to know.

Some sort of expression passed over her face. "No, I'm not." She looked at me steadily. "I'm not leaving you to deal with this on your own, Captain Hastings." She said firmly. "If you're thinking about me making my living, I can keep myself for a while. I haven't been extravagant in the past, and have some savings, as well as the money Mister Poirot left me."

I didn't like the idea of her using the money Poirot had given her to keep herself while we spent our time trying to solve his murder. "I can pay you while you're working here." I said.

"There's no need-"

"Yes, there is." I said firmly. It suddenly seemed incredibly important to me that she shouldn't lose out while we were investigating this case. I was used to keeping myself on less than £1,000 a year, so I was confident that I could pay her wages for however long it took for us to solve this case. A tiny voice at the back of my head wondered what we'd do if we never solved the case, but I ignored it; I couldn't bear to leave this case unsolved.

"Captain Hastings-" Miss Lemon began, the tone of her voice suggesting we were about to have a heated discussion about this. The telephone rang, cutting her off. "We'll talk about this later," she said firmly, and picked up the receiver. "Whitehaven Mansions. Yes. Oh, hello ambassador. Yes, he's here. Hold on for one second." She covered the mouthpiece. "It's the Belgian ambassador." She told me.

"Thanks, Miss Lemon." I took the phone from her. "Yes, Ambassador."

"Captain Hastings? I hope I'm not disturbing you." The ambassador's voice was anxious. "Do you have any information about our crime?"

I winced inwardly at his words. "We've made some progress." I said.

"But no information about the plans yet?"

"No, I'm afraid not." I admitted. "We may have some information on who stole the documents-" I began.

"You know who stole them? Then surely they can be arrested, Captain Hastings." He interrupted.

"It's a little more complicated than that, ambassador." I said. "I believe they were just as much a victim in this crime as you are."

"I don't understand, Captain Hastings. If they have the documents-" he broke off, his voice sounding slightly muffled as he spoke to someone in the room. "Yes, hello? Oh, yes, Hugo; I'll be there shortly." He shifted his attention back to our conversation. "I'll leave things with you for now, Captain Hastings, but I would appreciate it if you could get back to me shortly. This matter is becoming quite pressing."

"I promise you I'm doing all I can, Ambassador. Goodbye."

I passed the receiver back to Miss Lemon as he hung up. With a sigh I unfolded the blackmail letters I'd been given and looked through them, as though this time I'd find a clue I'd somehow missed before. "Is it possible to tell what sort of typewriter was used to type these letters, Miss Lemon?" I asked impulsively.

She raised her eyebrows. "I believe the police can match a letter to a certain typewriter if they have the machine, but I don't think so otherwise. Admittedly some typewriters are better than others." She held up a hand for the letters. "May I?"

I handed them to her and watched while she scanned the lines of type. "Hmm. A standard machine I'd say. Decent enough quality, although maybe aging now. Look, you can see where the E key is starting to type incorrectly – there's a slight line at the left where the left side of the type bar is hitting the paper hard enough to leave a mark while the right side of the letter is slightly faint." She tilted the letter towards me. "Probably the type bar is slightly bent and the key has a habit of sticking; it can be re-aligned if you're careful."

I craned my neck to see where she pointed at the letter; it didn't look that bad to me, but I trusted her word on typewriters. "So now all we have to do is test all the typewriters in London, and when we find one with this problem we'll know we've found our blackmailer." I said facetiously. I took the letters back from her and looked again at the instructions for delivering the documents. "J Smith & Sons general store." I murmured to myself. I straightened up. "Fancy a walk, Miss Lemon?"

"Of course. Where to?"

"To see the owner of Smith and Sons general store and sub-post office. I think I need to open a PO Box."

She gave me a smile. "Just let me get my coat."

J Smith and Sons was exactly the kind of shop I'd pictured: a modest, old-fashioned shop with a promotional display of absurdly large tins of goods and a giant Bryant and Mays matchbox in the window. I almost smiled at the 'and post office' written in small lettering under the main shop sign.

Inside the shop was fitted with the dark-panelled trappings of the general stores I remembered from my days as a young man. The counter, with years of polishing attempting to smooth over decades of wear that had left nicks and dents in the wood, fairly shone. The cash register and scales were likewise old but polished until they gleamed; I fancied even Poirot would have approved of the cleanliness of the shop, and the neat lines of the jars of sweets and the cigarette packs on the shelves behind the counter would have appealed to his love of 'order and method'. The young woman behind the counter gave me a polite smile. "Good morning, sir."

"Good morning." I glanced at the jars of sweets behind her. "Could I have a quarter pound of bulls-eye please?"

"Of course." I watched as she poured the required amount of sweets into the scales, feeling in my pockets for change. "I notice you have a post office here."

"That's right, sir." She gave me another quick smile as she tipped the sweets deftly into a paper bag, twisting it shut with practised ease. "That'll be tuppence please."

I laid the coins on the counter. "Would it be possible for me to open a PO Box?" I asked.

She looked apologetic. "Oh no – I'm sorry, sir, but we don't offer that service here."

"You don't? But-" I caught myself before I blurted out the wrong thing. "I see; right."

"What is it, Mary?" A white haired old woman appeared at a door to the back of the store. She peered short-sightedly at me.

"Just this gentleman, mum. I'm dealing with his query." There was a slight edge to her voice as she spoke to the woman. "I am sorry. They have bigger post offices in town that could help you."

I managed to recover myself. "I expect so. Well, thank you anyway."

Outside Miss Lemon looked up at me expectantly. "What happened, Captain Hastings?"

"What sort of blackmailer would open a PO Box in a general store and sub-post office that doesn't offer a PO Box service, Miss Lemon?" I asked.

"They don't?" Miss Lemon sounded as surprised as I was. She was silent for a moment. "I don't understand this case, Captain Hastings." She admitted.

I sighed. "I don't think I do either, Miss Lemon." I opened my paper bag and held it out to her. "Bulls-eye?"

I was jerked awake from a confusing dream in which Poirot was explaining that a moustache to provide perfect facial symmetry was essential for solving a murder and lay for a moment trying to collect my wits before I realised I'd been awakened by the shrill ringing of the telephone.

It was still dark outside, and I stumbled out into the living room, flicking on the light, wincing at the harsh brightness before picking up the receiver. "Hello?"

"Captain Hastings? It's Japp."

"Oh. Hello, Japp." I stifled a yawn and groped for my watch. Almost half past four. What on earth was Japp ringing me for at this time of day?

"Sorry to awaken you so early, but I thought you'd like to know the news." Even through the phone I could hear the satisfaction in Japp's voice. "We've got Poirot's murderer."

His words brought me awake like a jet of cold water in my face. "My God!" My fingers clenched tightly on the receiver. "When?"

"A few hours ago. I'm currently at the morgue in Scotland Yard if you want to join me."

"I can get there in about half an hour."

"That's fine. I'll see you then, Captain Hastings."

After Japp had rang off I spent a few seconds debating whether or not to call Miss Lemon. While my natural instinct was to shield her from anything unpleasant, I knew that she had worked in a morgue during the war, and besides, she'd never forgive me if I went without her. I gave a sigh and started dialling her number.

"We received reports of a gunshot last night and arrived at the scene of a suicide who'd shot himself. But when we ran a test on the gun, ballistics revealed it matched the weapon that had been used to kill Poirot." Japp announced, opening the door of the morgue in the bowels of Scotland Yard. The room was cold, tiled, and smelt strongly of disinfectant.

He hesitated by the table. "Are you sure you want to see this, Miss Lemon? It's not a pretty sight."

"Quite sure, Chief Inspector," she answered. Her voice was cool and composed.

He tilted his head slightly in acceptance and pulled the sheet back from the body. I don't know about Miss Lemon, but I felt a jolt of shock almost like a physical blow in the stomach as we gazed down at the pale, still face of Michiel Boulet.