"How extraordinary!" gasped John, looking out of the window of 221b onto the street below. "Someone's left a trail of red rubber bands down the street."

He turned to his flatmate. "What do you think that means, Sherlock?"

"Means?"

"Yes. Do you think it might be a secret signal left by an ancient occult society to alert their members to strange goings-on?"

Sherlock snorted. "You've been watching too much crap telly. No, it means that the postman's just gone down the street."

At that moment, they both heard the clatter of the letterbox downstairs in the hall.

"Told you!"

John shook his head in amazement. "Brilliant! It's uncanny! I don't know how you do it. Right, I'm expecting something from Amazon, so I'll just go and pick that up."

He was disappointed, though, when he reached the foot of the stairs. There was, indeed, a bulky envelope on the mat, but it was not addressed to him. Or to Sherlock. Or to Mrs Hudson. Or to the married ones next door. It wasn't even addressed to anyone in Baker Street.

"Sherlock," he asked, scratching his head, as he reentered the room, "the postman's just delivered a letter addressed to 185 North Gower Street. What's going on?"

"I'm not sure," Sherlock conceded. "I'd say that was probably more your area than mine."

"My area?"

"Yes."

John stared back at him blankly.

"So, what do you reckon? Cataracts? Wet macular degeneration? Or just plain, boring old myopia?"

"Ahhh!" exclaimed John, after thinking about it for a moment. "I see! The postman. He's partially sighted."

"Well, let's see what's in it," said Sherlock, grabbing the package out of John's hands. "I love parcels!"

"You can't do that!" John's face went a little bit red and a vein began to pulse angrily at the base of his neck. "It isn't addressed to you!"

"Aw, John, you're so sweet. So endearingly bourgeois," muttered Sherlock, as he ripped open the top of the envelope and tipped its contents out onto the table.

They both took a sharp intake of breath. The envelope contained no card or note, no exciting present. All it contained was five banana skins and a dried flower.

"But that's just mental!" protested John. "What kind of a lunatic would send five banana skins through the post?"

"Some ancient secret societies used to send five fruit pips as a warning or a threat," Sherlock explained in a voice that had, all of a sudden, taken on an extremely grave tone. "This is probably the modern equivalent."

"Oh, come off it! How is this the modern equivalent of five pips?"

"This is the twenty-first century. Nobody eats fruit and vegetables any more. Not unless they're being forced at gunpoint by Jamie Oliver."

"But a banana is a fruit!"

Sherlock's expression faltered. "Is it? Oh, bugger. I told you my general knowledge was pants. Next you'll be telling me that the sun doesn't revolve round the earth, or something."

"And what's this dried flower all about? And why's it been dyed this ridiculous colour?"

All the colour suddenly drained from Sherlock's face.

"It's the Turquoise Thistle!" he whispered in an awestruck tone.

"The Turquoise Thistle?" repeated John, furrowing his brow.

"Oh, bollocks, why's it always me that's landed with the clunking exposition, while you just get to ask stupid questions and look a bit dim, but cute? Yes, the Turquoise Thistle! An ancient Scottish crime network with fingers in a number of criminal pies all over Europe…"

"Is there such a thing as a criminal pie?"

"You've obviously never eaten at Speedy's."

An ominous thought occurred to John.

"If what you're saying is true, aren't we wasting time standing here chatting? Shouldn't we be hot-footing it to North Gower Street to warn this…" he consulted the envelope "…this Donald McTrousers that a murderous horde of Scottish gangsters are out to get him?"

"Good point! You get the coats, I'll look for a taxi."

But when the taxi pulled up in front of 185 North Gower Street, which for some reason looked freakishly familiar to them, they realised that they were already too late. The front door was hanging open and a turquoise thistle had been carefully placed on the doormat.

"Damn it!" sighed Sherlock, in frustration. "We just missed them. They've already been and gone. I must say, they're rather overestimating the Royal Mail if they expected the warning to arrive before they did. They only posted it four weeks ago. And they sent it second class!"

"That's a point," said John, squinting in puzzlement. "Why would they send a warning, anyway? Doesn't that defeat the object? I mean, if you're going to murder someone, why would you bother telling them that you were going to murder them? Isn't that just giving them more of a chance to run away?"

"Ah, the Scottish mind is totally inscrutable," said Sherlock, sadly. "It is not for us to fathom their arcane and impenetrable ways. Come on, let's go up and examine the body."

"The body?"

"There's bound to be a body. The Turquoise Thistle never leave without getting what they came for."

And, indeed, when they entered the first floor flat, the first thing they saw was a dead man lying on the floor, dressed in a kilt, a dirk sticking out of his chest, blood seeping out all over the frilly front of his shirt.

"The murderer obviously got in through the window," said Sherlock, with total confidence.

"How do you know?"

The detective pointed to a dark purple box of chocolates lying on a table beside the body, with a card attached – no writing, just the black silhouette of a man against a white background.

"And all because the lady loves Milk Tray…"

"But he's not a lady."

Sherlock shrugged. "Well, he's wearing a skirt."

Then, without warning, the great detective dropped to his knees and stuck his head up the dead man's kilt.

John was shocked to the core. "Sherlock! Stop it! That's just wrong! I must admit, I've always wanted to know what a Scotsman really wears under his kilt, as well, but now is not the time!"

A muffled voice emerged from under the kilt. "No, John, no! I'm not perving. I'm looking for his tattoo…Ah, there it is!"

Sherlock threw back the kilt triumphantly to reveal a small tattoo of a thistle on the dead man's member.

"Ouch!" said John.

His friend beamed, smugly. "All the Turquoise Thistle have them. And in that place, too."

"What about the women?"

"I don't know," conceded Sherlock. "I've never been up a woman's skirt. Not really my area. But they also have the tattoo somewhere. Now, come on, we need to look for clues before the police get here."

They scoured the flat from top to bottom and found a number of clues which gave them some pointers to the man's character. By the time they had finished, they had ascertained that he was a very dour sort of person, that he ate a lot of porridge, drank a lot of whisky and that he didn't like spending money if he could help it. But they found nothing which told them who the killer was or where Donald McTrousers had been in the days leading up to his death.

At least, not until Sherlock's trained eye lighted upon a book, the People's Friend Bumper Storybook 2010, which was lying on the mantelpiece. He flicked open the cover and saw a slip of paper gummed in the front with a column of dates stamped on it.

"It's a library book," he observed, doing a quick calculation in his head, "and he must have got it out a week ago."

John shook his head. "No! Camden Libraries only lend their books out for three weeks at a time. The last date is three weeks from now, so he must have got it out only this morning!"

Their eyes met.

"Do you think it's relevant?"

Sherlock shrugged. "We've got nothing else to go on."

They ran a couple of blocks up the Euston Road to St Pancras Library.

John had been foresighted enough to make a note of the Dewey classification number of the book, so it was easy enough to trace Donald McTrousers' steps through the library and find the exact spot where the People's Friend Bumper Storybook would have been shelved, but they found nothing suspicious on their travels. It seemed to have been a wild goose chase, after all.

"Aagh! What a waste of time!" groaned John, thumping one of the shelves in frustration.

To his embarrassment, the shelf promptly collapsed, sending books flying in all directions.

"Sssh!" hissed about six librarians, each putting a finger to his or her lips and glaring at him disapprovingly.

He mouthed a sheepish "Sorry!" in their direction and set about picking up the books he'd just dropped.

He noticed after a few moments that Sherlock was doing absolutely nothing to help and he was about to have a go at him about his unbelievable laziness, when he noticed that his impossible flatmate was staring at something on the wall, where the shelf had been. The sign of a thistle had been graffitied on the wall in turquoise paint, just behind the shelf where Donald had found his library book.

"Right," said Sherlock, trying to get things straight in his head. "It's all fairly straightforward. The Turquoise Thistle, drawing on their fiendish, esoteric powers, must have psychically discovered, by long-range mind-reading, not only the fact that McTrousers was going to go to the library this morning, but also the exact shelf that he was going to borrow a book from. So all they had to do was break into the library in the middle of the night, remove all the books from that shelf and spray the thistle symbol on the wall, before replacing all the books again.

Then, when Donald arrived here this morning, instead of just taking one book off the shelf, like a normal person would, he must have pulled out the whole shelf of books, in order to be able to see the symbol sprayed on the wall. That would have struck such terror into his heart that he would have fled for his dear life, stopping only to wait calmly in line to have his library book scanned and stamped (obviously a high priority at a moment like this), before returning to his flat, the one place in all London where the Turquoise Thistle were bound to find him."

John nodded. It seemed like a watertight, plausible theory to him, too.

Sadly, however, it took them no further forward. They still had no idea why the Turquoise Thistle had killed McTrousers, who the killers were or where to find them. They needed to do some more old-fashioned detective work.

Their investigations took them deep into the Scottish quarter of London – a little-known, sinister area where the narrow streets were lined with cafes selling haggis, neaps and tatties, clootie dumping and deep-fried Mars bars, where the mournful sounds of bagpipe music filled the air, where most of the men were wearing kilts, Argyll sweaters, Bay City Rollers socks and tam o'shanters, where an English face was met with hostility and suspicion and, if you looked at anyone the wrong way, they were as likely to give you a Glasgow kiss as to spit in your Irn-Bru.

They decided to browse round a tacky gift shop and see if they could learn anything there. The sign above the shopfront proclaimed its owner to be Moira McStereotype and its windows were crammed with plaid shawls, Andy Stewart CDs, golf memorabilia and books of poetry by Rabbie Burns.

"Och, aye, dunooo!" called the woman behind the counter, almost as soon as they had walked through the door. "Hoots, mon, why d'ye nay buy a wee tin of shortbread to give to your wee lassie?"

She brandished a garish tartan-patterned tin with a badly tinted colour picture of Edinburgh Castle on the lid in front of John's face.

"Well," he thought, "Sarah has got a bit of a sweet tooth and she might like it…"

He was just rummaging in his pockets for some change, when Sherlock slammed the dried thistle they had recovered from McTrousers' flat down on the counter and asked, in his most menacing tone, "Do you recognise this?"

"Och," said the woman, frowning, "he dinnae normally send a Sassenach, though."

But she shrugged, went into the backroom for a minute, before returning with a massive package of pinkish-orange fish, which she was on the point of handing over to Sherlock, when the bell on the door tinkled as a group of armed police officers came hurtling into the shop, led by a dishevelled figure in a raincoat who held out his warrant card, yelling, "Freeze! Detective Inspector Lestrade of Scotland Yard!"

Moira McStereotype looked away into the middle distance and her face blanched to the colour of whey. "We're doomed!" she cried "DOOMED!"

"Wrong programme," John corrected her.

She looked bitterly disappointed.

"It's MUDDER?" she suggested, tentatively.

"Well, no, it's not that one, either, although you are a bit warmer this time," John added, in encouraging tones. "It does have the name of a detective in the title, but this one's still in the programme, though, not one who died about ten series ago…"

"Moira McStereotype," interrupted Lestrade, "I am arresting you on suspicion of handling poached salmon. You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something which you later rely on in court. Anything you do say may be given in evidence against you.…"

"Thanks for the tip-off, Sherlock," Lestrade said later, when McStereotype had been led away in handcuffs. "Without your help, we'd never have smashed this salmon-poaching ring. We knew that illicit smoked salmon was pouring into the country from over the border and being distributed somewhere in the Scottish Quarter, but we'd never been able to locate the drop-off point, until now."

Sherlock looked deeply unhappy.

"We moved too early, though. We didn't manage to catch the big fish."

Lestrade looked down at the vacuum-packed salmon on the shop counter.

"Oh, come on, mate! It's a pretty big fish!"

"No, not THAT big fish! I mean the brains behind the operation, the Scottish criminal mastermind who has planned all this and pulled all the strings….oooh!"

Sherlock was pulling his Frankie Howerd face again.

"Moriarty! I bet it's him!"

"Er, isn't Moriarty Irish, though, Sherlock, not Scottish?" John pointed out.

Sherlock looked a bit put out. "Is there a difference? Oh, well, he's one of those brooding Celtic types, with an accent – they're all the same to me."

He glanced down at the fish. "Mmmm, now all we need to find is some bagels and cream cheese…"