John had often wondered about the cab drivers who shunted Sherlock and himself between cases. How much had their cabbie heard and understood that evening five years before, when Sherlock had taken his phone and deduced half his life from it? What had the dozens of others since then made of it, when their passengers had suddenly struck up a back-seat conversation about intestines and land mines and dominatrices?

Well, cabbies were paid to be discreet, he reasoned. Anyway, most of them had probably had worse passengers than the two of them discussing gory crimes. Ones who got up to all sorts in the back seat, or threw up back there coming home after a bender.

He thought wryly of the event that had more or less cut short his honeymoon – Molly vomiting, mostly on him, in the back seat of that cab in Paris. She could hardly be blamed for that one, with Charlie on the way. His thoughts reached the nagging anxiety that had been plaguing him for weeks. We only wanted one more. How are we going to manage twins?

He cleared his throat. "So he's innocent, then?"

Sherlock had been lost in thought, his chin resting on his hand. At this he stirred and looked across at him. "Of course he's innocent," he said. "What sort of a perpetrator would make up something so stupidly incriminating as his leaving a teenage girl to her fate, because it never occurred to him to escort her home for her own safety?"

John frowned. "Yeah," he said slowly. "That… was a bit weird."

"No, it wasn't." Sherlock turned back to the window. "He left her there because he was expected home, but she never expressed a desire to leave."

"Why not?"

"Do think this one through."

"… Because she was planning on meeting someone else there," John faltered. "The person who killed her?"

"Maybe," Sherlock said. "Maybe the person she intended to meet came across her corpse some time later and just left her there, with no word – they didn't want to get involved. People don't, you know. Or maybe she intended to leave the castle alone and travel in the opposite direction to Matthew, for some reason."

"The perpetrator," John said. "At the crime scene, you gave a profile… but that was just based on who she might have slept with, wasn't it? Good looking, not too much older, smelled nice. Not exactly your most amazing deduction."

Sherlock smiled for a second.

"But she slept with Matthew, and you reckon he didn't do it," John went on. "And Celeste's parents said they were expecting her home last night, so what was she playing at, meeting someone there when she should have been on her way back?"

"Celeste's parents didn't report her missing until midday, and she'd already been dead at least twelve hours by then," Sherlock pointed out. "Obviously, she's made enough of a habit of staying out that they weren't alarmed when she didn't return for her curfew. I'm sure Harry always came home on time every night she was expected to."

John smiled wryly. "Harry turned climbing out of windows in the dark into a fine art form."

"I can imagine."

John looked down at his notepad, as if puzzling out his own notes could advance the case. Sherlock sometimes cracked things wide open by staring for hours at a collection of clippings thumbtacked to the wall at Baker Street. But he wasn't a consulting detective, and he hadn't written down anything of great worth. Just useless scribbles, really, like left at 8pm and broken boards. "And now this new one," he said. "This new victim. Male, late forties. Completely different victim to Celeste, but they think it's another one from the same killer?"

"Clearly the same MO."

"Another note, then." John exhaled. "And the last one was handwritten. Are they analysing it?"

"Standard procedure. But it's unlikely to give us too many useful leads," Sherlock said. "Handwriting analysis is generally only useful if one has something to compare it to."

"Serial killers usually stick to the same kind of victim, don't they? And why all the secrecy from Greg, anyway? It'll be on the news tomorrow morning, if the media are doing their jobs."

Sherlock shifted in his seat. "You don't lip-read, do you?"

"No." John sighed heavily. "But I've got a feeling you're about to tell me that – "

"I do. And as we walked out the door, Sergeant Alan Peters asked Merivale what was going on. I saw, rather than heard, her response."

"What was it?"

"'One of Lestrade's boys.' Given Matthew is the only son he has, and we left him safe and sound at the station, the newest victim is a member of Lestrade's team."

"Shit," John groaned into his hand. As if Greg hasn't got enough to worry about. "Which one, do you think?"

"You just said it," Sherlock said. "Male, late forties. Only one regular member of Lestrade's team fits that description."

John thought this over for a few seconds. "Oh, God," he finally said. "Bob Thompson."

Sherlock had always said it, right from the beginning – each murder, and each murder scene, was as distinct and unique as a fingerprint. But the little white house they eventually pulled up at, now festooned with yellow police tape and flooded with harsh spotlights, seemed just like many of the other crime scenes John had attended in the last five years. There were a lot of unfamiliar faces about, though, since he'd never worked with anyone on Merivale's team. On getting out of the car he caught a glimpse of Philip Anderson in the open doorway, silhouetted against the light.

"Well," he said to Sherlock, gesturing. "At least we've got Anderson here."

Sherlock grunted, but there wasn't time to comment on Anderson's value as a forensic tech before he came forward and met them on the front step.

"Gear up," he said coldly, without any form of greeting. "All of it, Holmes."

Just as Donovan had defaulted to Genius, Anderson was now in the habit of addressing Sherlock as Holmes. John had never remarked on it, but in some ways, it sounded worse than Freak. Real public-school stuff, conjuring up a past world of wealthy, privileged bullies, with all the morals of a pack of wild animals. Anderson wasn't going to shove Sherlock face-first against a wall or hold him down under a scalding shower, but he'd sure as hell had fun smirking at the regulations he now had to follow.

Sherlock hissed, annoyed, but he'd been through this routine before. Neither Anderson nor Merivale were going to allow him past the threshold without his having full protective gear on. He looked back at John uncertainly, then took off his coat and scarf and gave them to the nearest PC.

"Mind those," he ordered. "And be careful with them." He took the protective suit Anderson handed him and started putting it on.

"Who's that?" John asked quietly as he did the same. A crying woman in her mid-forties was sitting in one of the squad cars; beside her, a policewoman was trying to put a blanket over her shoulders.

"Widow," Sherlock said bluntly. "Or close to it. No wedding or engagement ring. They're either separated or divorced, but she probably found the body - "

"Sherlock," Anderson said.

Sherlock looked up at him.

"Does Greg know about this?"

"Not yet, but it will only be a matter of time." Sherlock zipped up his suit and reluctantly took a pair of gloves from him. "They may even have someone out there tonight with the news."

"Well, if you want my advice – "

"I don't."

"You'll watch your mouth around here," Anderson went on, undeterred. "These officers – they're in shock. They always are when it happens to one of their own. We need your help, but if you say the wrong thing in front of the wrong person, they're going to punch your head in for it."

"Come on, Anderson," Sherlock said, smiling wryly. "You've not noticed that we only ever bury saints?"

"For the time you're here at this crime scene, Thompson was a saint… Detective Inspector Merivale," he said deferentially as she approached into earshot. "Just prepping these two. We're acquainted."

"Yes, I know." Merivale's tones were so bland that almost nothing of her opinion of Anderson could be read from them. "And I'm told you two gentlemen are also familiar with this sort of violent crime scene."

"Uh, yes," John muttered, glancing over and seeing the disdainful expression on Sherlock's face. "Former army, in my case."

"Then you know what it's like to lose a fellow soldier, Dr. Watson," Merivale said.

John nodded.

"I didn't know Thompson, but some of the others here did. Remember that. I haven't had a look yet, but I'm told it's not a pretty sight in there."

DS Robert Thompson, aged forty-eight, lay sprawled on the kitchen floor on his back, one leg bent under the other at a hideously unnatural angle. Something viscous and sticky was splashed all over his head and face, and a swarm of flies writhed over his open eyes and in and out of his nose and mouth. His swollen, grey tongue poked out from between his encrusted lips.

A large white rectangle of paper was pinned to the front of his shirt. From the other side of the room John made out aggressive black lettering, just as the paper fluttered up on itself and hid them again.

John looked at Merivale. "I won't touch anything," he said. "Normally I just have a look…"

She shrugged. "Well, go on, then," she said, "have your look."

Flicking aside Merivale's contemptuous tone, John carefully edged forward to the body. At any crime scene supervised by Lestrade, he'd not had to keep "having a look" strictly literal, so long as common sense prevailed. But the thought of how Greg was going to react to one of his closest colleagues being murdered checked any temptation to try Merivale at her threat to remove both Sherlock and himself from the crime scene. Merivale may have told Sherlock he wasn't going to be Lestrade's man on the inside, but as people loved to remind him, he wasn't Sherlock Holmes.

He folded his arms against the urge to tweak Thompson's head to one side for a better look at his neck, or lift the paper pinned to his shirt.

"Judging from the looks of him, he's been dead at least thirty-six hours," he finally said. "Maybe as many as forty-eight, but I wouldn't think that much in this sort of weather." The August day had been warm, and even so late as it was, the breeze coming in through the half-open kitchen window wasn't chilly. "And we're only just finding him now?"

"Lives alone, and had the weekend scheduled off, I imagine. More importantly, it looks like he was already dead when Celeste Biondi met her killer." Sherlock dropped down on his haunches beside him and sniffed deeply. John recoiled. The room smelled like a lot of things, and none of them were pleasant.

But now he thought about it, there was an odd smell underneath the stench of faeces and urine and sweat and two-day decay. Something sour that he couldn't place.

"Cause of death?" he heard Peters ask over his shoulder.

He smiled grimly. "Impossible to say just by looking at him," he said. "But you see he's got cyanosis around the mouth, and I think the hands. Could have been asphyxiated, but there are no marks on his neck, and I think the note might be spot on for this one." He gingerly uncurled the paper on the dead man's chest to re-read the stark, swooping black words written on it.

What pain it was to drown!

"And not difficult to see what he drowned in," Sherlock remarked practically, rising to his feet and walking over to the sink. "The sink is still full of it." Bending forward so that his lips almost touched the stainless steel, he sniffed again. "Claret, mainly, if I'm not very much mistaken."

Now John recognised the odd smell that Sherlock, no doubt, had placed immediately. Red wine, of course. It was the vestiges of wine on the dead man's hair and face that had attracted every summer fly in London into the room. He glanced over at the open window, then back at the note pinned to the dead man's shirt.

No wonder there was a very young uniformed PC standing in the front hall who, while neither crying nor vomiting, sounded like he wanted to call his mother.

"I'll make it easy for you all." Sherlock turned back to face the room. "This is a quote from Shakespeare's play, Richard III. In context, it's a line from the Duke of Clarence, referencing a dream he had. Later in the scene, he is executed on his brother's orders, in a butt of Malmsey wine."

By now Sherlock had attracted the attention not only of Merivale, but of Alan Peters and several other attending officers. Anderson, too, stood in the doorway listening in.

"Okay," Peters said. He was a thin, sharp-nosed man in his mid-thirties, with temples that bulged through his thinning dark hair. "So what's that got to do with Bob Thompson?"

"Some sources indicate that, traditionally, a pig's head was pickled in each butt, or cask," Sherlock told him. "Rather an appropriate death, then, for a police officer who was very clearly an alcoholic - "

"Actually, Sherlock," John said, "I'm surprised Peters didn't hit you harder for that one."

"Oh, shut up," Sherlock growled, his voice muffled through the tissue John had offered him for his bloody nose. The pair of them stood on the street-lit kerb outside the Thompson house, awaiting the arrival of a cab to take them home.

"No, you shut up." John gestured to the house behind where the police were still carrying out their investigation. "That was our chance to help Greg and Matthew, Sherlock, and you blew it because you couldn't resist being a smart-arse."

"You don't suppose the victim's being a drinker is relevant when he was drowned in a sink full of wine?" Sherlock retorted. "If Peters would like to tell me how I can solve this crime without deducing anything unsavoury about the victim's private life, he's more than welcome to."

"Well, if it's any comfort," said a voice behind them, "I've just got Merivale to stand Peters down for putting your DNA all over a crime scene, Sherlock."

The both turned. Anderson had followed them out and stood a few feet away. He was still in his protective suit, though he'd pulled down the hood and taken off the shoe covers.

Sherlock was just then wholly absorbed in finding a bloodless spot on his tissue, and said nothing.

"And Gifford's on her way in," Anderson continued. "She's better at this than I am." He took a few steps closer and glanced over his shoulder, as if worried about being overheard.

"That's very comforting," Sherlock said dryly.

"I've heard they've arrested Lestrade's son for the murder of the girl," Anderson ventured.

"Not arrested," John said, seeing that Sherlock wasn't going to answer. "At least, not yet. But it's not looking good. He was the last person to see her alive." He wondered briefly how Molly was getting on with Celeste's post-mortem, and glanced briefly at his watch. It was getting late.

"Well, this surely puts a spanner in the works on that theory," Anderson said. "Did Matthew even know Thompson?"

"I've no idea," John said. "I'm sure they'll ask him."

"And just what, exactly, is your interest in all of this?" Sherlock interrupted, icily polite.

"Not academic," Anderson said.

Sherlock snorted. "Oh, what a surprise."

"I consider Greg Lestrade a friend, if you really must know." Anderson glanced down at his shoes.

"A friend?" Sherlock raised one eyebrow. "I don't think he's ever mentioned you as one of his."

"No, but he wouldn't, would he?" Anderson said, suddenly eager. "When you faked your death, I nearly lost my job over it. But after he got back from his own suspension he put in a report to DCI Chambers, praising my past work and recommending they keep me on." He paused. "Do you get it? He thought it was my fault you killed yourself. He told me that, just after it happened. And then he helped me anyway."

"And you want to repay the favour by helping convict his son of murder."

"Oh, don't be stupid, what reason could he possibly have for doing this?" Anderson screwed up his nose in disgust at the idea. "And that's the other thing. Greg's son, one of Greg's close colleagues. Fill in the blank. Whoever did this might come for him next."

"The thought had occurred to me," Sherlock said shortly.

Anderson checked over his shoulder again. "You're going to have to work hard for Merivale to overlook this one," he said, dropping his voice. "I told you to keep your mouth shut – just a second, Sherlock, before you get clever. You two got kicked out of this, but I didn't. Everyone forgets the techs. We're not as visible as the detectives."

Sherlock shrugged. "So?"

"So don't you see? I can help you," Anderson said. "And don't pretend you don't get people to help you, and not just John or Greg. You were out interviewing this afternoon, weren't you? With Donovan and Jones."

"Oh, she's 'Donovan' now? I'm relieved to find your affair with Sally is over," Sherlock remarked. "I imagine her husband prefers it that way."

"Do you want my help or not, Holmes? Because I doubt you're going to be able to access crime scene details any other way."

Sherlock shut his eyes and exhaled. Beyond, at the end of the street, the headlights from a cab bounced into view.

"Baker Street," he finally said. "As soon as you can meet us there. Don't text me. If anything ever comes of it, they may audit your phone."

Anderson nodded. "I've got to get back," he said as the cab pulled up at the kerb beside them. "Oh, and by the way, if anyone asks, I just abused both of you for contaminating my crime scene."