Hayley wrung her long fair hair in her hands and over her shoulder as she hurried down the forecourt stairs of the British Museum. No Matthew – at least, no Matthew anywhere in the Ancient Greek display area or anywhere in the Reading Room. She'd just been contemplating where to try next when Mel had texted: Sorry, your dad wants to come get you. Be out front half hr?

She glanced at her watch. Still a couple of minutes before Mel's deadline. God, she really needed a cup of coffee.

Traffic on the street outside the main gates was sluggish at best, but she gave a perfunctory glance both ways before stepping across to the Starbucks on the other side of the street. Ten minutes later, hot styrofoam cup in her hand, she stepped back out again, backing up for a second as a cloud of cigarette smoke wafted into her face from a young man who'd just strolled obliviously past. Wrinkling her nose in disgust, she pulled out her phone and scrolled down to Jake's number.

"It's me," she said as soon as he picked up. "He's not in the museum. Well, he's not where I thought he might be in the museum. You could probably lose someone for a week in there, but none of the staff seem to remember him. Did you have any luck with the cab companies…?"

"Not yet," Jake said. "I've got two of them reviewing their footage now, but nobody who was on shift in the area last night seems to remember him. But we'll find him, Hayley. We will."

Hayley hesitated. "I can't help thinking," she said, swallowing hard. "About what you and Dad were talking about last night, about this being aimed at Dad. Do you think it is?"

"… Maybe," Jake conceded, puffing out a breath into the phone receiver. "And if someone's got it in for your dad, after his family or his team or both…" He trailed off, leaving the obvious conclusion to hang on the line between them.

"After you're finished work, pack up some things and come over to the house for a few days." Hayley shifted the too-hot cup in her hand. "Just until you get this guy. Dad won't mind. I think he'd like having you around, actually. He likes you a lot more than you think."

He hesitated. "But my parents - "

Hayley smiled grimly. She'd anticipated that line a mile away. Jake was devoted to both his parents and his brother Josh, who was thirteen. "If someone's targeting dad, then they're not going to be interested in Josh or your parents," she explained, stepping off the kerb on her way back across the street. "You'd be making them safer by getting out of the way. And anyway, if… oh, hell," she broke off, pausing for a second to listen to the faint, insistent little bleep on the line. "I've got to go, Jake, there's another call coming in and it might be Matty. I'll call you later, okay?"

Without waiting for a response, she switched lines. "Hello?"

Silence. Hayley put her finger in her free ear to concentrate better on the line. A bus roared off from the kerb and she winced, looking around for a more quiet spot to head to. "Hello?" she said again. "Matty, is that you?"

For a second she heard, or thought she heard, a hiss of breath.

"Matthew?" she insisted. "Dad's going nuts, you know. I know you're scared, but running away isn't going to solve anything. If you tell me where you are I'll come and get you…" Trailing off, she looked up to see her father getting out of the car on the other side of the road. She hailed him with one hand.

"Look, come on," she said stridently. It had just occurred to her that she may not have been speaking to her brother after all. "You've gone to all the trouble of ringing me, you may as well actually talk. Who is this?"

Silence. Dad had just approached her and stopped short, watching anxiously; she pointed to the phone silently and he nodded, then pulled his own phone out and fed in a number, wandering away a few steps. She knew he'd just called someone he knew in technical forensics to see if they could trace the caller. Keep them on the line, he mouthed to her, and she nodded.

Sherlock stepped out of a clump of hawthorn and spindle, looking around warily. After a few seconds he straightened up.

"Nobody here," he announced. "They've already collected their forensic evidence. Still, I expected a police guard of some sort. Follow me, and keep alert. Remember what I told you."

Following Sherlock to the boarded window, Matthew nervously ran over what Sherlock had told him. In the event of an emergency, he was to do exactly as he was told and, if separated from Sherlock, he was to make his way back to the Cock Lane bolt hole and stay there until further notice. Following Sherlock's example, he clambered through the gap in the broken boards and into the dark space beyond. A large room, octagonal, and lit dimly by the cracks of daylight through the boarded windows and a high garret roof. He coughed as dust particles swirled up into his face.

"So you took Celeste here. Did you go upstairs?" Sherlock gestured to the staircase, a winding, steep affair of rusted wrought iron. Matthew shook his head.

"No," he said hoarsely, giving one last cough. "That's… part of what I don't understand. Why she went up. We had a look at the stairs and thought they were dangerous. If you put a foot right through it…"

"And yet, at least two people made their way up here quite easily," Sherlock remarked, looking down at the first three steps. "On the left, here. These footprints are mine, and these are your father's, from where we both went up yesterday. Indistinct. No details of the treads. We were both wearing shoe covers. But here." He pointed again. "Here's a tread-mark. What size shoe do you wear?" He glanced down at Matthew's feet. "Ten," he answered for him. "These are a nine. Very casual shoe. Trainers, probably."

"Can you see Celeste's shoeprints?" Matthew asked him. "Or was she… was, I mean…?" He swallowed. Sherlock, down on his haunches at the bottom step of the staircase, drew his slide magnifying glass out of his pocket with one hand and gestured to Matthew with the other.

"What do you think?" he asked as Matthew crouched down beside him.

Matthew gave him a doubtful look. Sherlock sighed.

"Can you just look - no, actually don't just look. Observe. What shoes was Celeste wearing when she died?"

"… I don't know."

"I do. And traces of their tread are here." He pointed. "Aside from that, I believe they found her fingerprints on the stair rail, which indicates she went up of her own accord and wasn't being carried. But that's not important," he went on without pause. "Do you know why I brought you here?"

Matthew shook his head.

"Because I need you to remember everything that happened in here with Celeste. Everything you did, everything you said. The more I think about it, the more I'm convinced that you're the link between Celeste and Thompson, not your father."

"But I didn't know Thompson."

"You didn't need to. I believe that both Celeste and Thompson were killed because they knew too much. And whatever they knew, you know it, too." He pointed for emphasis. "Your book. You've been researching it for over a year."

"Uh, yes." Matthew scratched the back of his head for a second in a way that reminded Sherlock unmistakeably of his father.

"And you took inspiration from real life cases."

"Yeah. I based the main detective off you," Matthew said. "His name-"

"Yes, congratulations on managing to give him a more absurd name than Sherlock Holmes." Sherlock rolled his eyes. "His name is not important. The book isn't due to be published until next month, which means that if you've kept to your publishing contract and managed to keep the contents off the internet all this time, very few people will have ever read it. Did Celeste?"

"Yeah." Matthew blinked. "Of course she did…"

"And you didn't base the book off anything to do with your father's team? Some case or operation that Thompson would have been involved in?"

Matthew shook his head. "Dad doesn't… well, he did help a bit with the book, but you can't get him to talk about actual cases. He says it's just asking for trouble where real people are involved." He ruminated for a few seconds. "But I did –"

Before he could get any further, they both heard the crunch of boots on the gravel just outside the window they had entered by.

"You what?" Sherlock said insistently, giving Matthew's arm a little shake. "Quickly, tell me – oh, here!" He grabbed Matthew by the wrist and dragged him to the battered door of a storage cupboard under the stairs. Pushing his head down with one hand, he opened it with the other and shoved him in. "Be quiet," he ordered. "Don't move."

"Hello?" A female voice emerged from behind the broken boards. "Is anyone in here?"

The storage cupboard, Sherlock had discovered too late, was far too small for both Matthew and himself. Left without a hiding spot, he walked toward the beam of Detective Constable Sarah Draper's torch. It bounced over the wall behind him and finally came to rest directly on his suit. He heard her little squeal of alarm and wondered, for a moment, how a woman who clearly had weak nerves could function as a police officer. He couldn't even imagine Sergeants Jones or Donovan squealing like a spooked six-year-old.

"Hello," he said flatly, heaving a sigh. "Sarah, is it? I believe we met yesterday."

The torch beam dipped violently. "Turn around!" she screamed at him. "Now! And keep your hands where I can see them!"

"Oh, for God's sake." Sherlock turned his back to her and paired his wrists so that she could cuff them.

She's terrified, he realised, feeling her icy fingers on his as she fumbled clumsily with the cuffs. It must be obvious to even the most dim-witted officers by now that any one of them could be next.

"Ow! Careful," he complained as she slammed one cuff down and it nipped his wrist.

"Sherlock Holmes," she said shakily, "I am arresting you for trespassing beyond a police barrier…"

"Fine," he said, cutting off her explanation that he had the right to remain silent. "Take me back to Scotland Yard. I need to speak with Detective Inspector Merivale – now."

June Merivale sipped at her first cup of coffee for the day – about three hours later than she preferred it – and groaned inwardly. Draper had just radioed in that she'd arrested Sherlock Holmes at Severndroog Castle and was bringing him in imminently, and Merivale was trying to decide whether she was going to ream Draper for attending the scene on her own now or save it for later. Before she could move her attention to the homicide report on her desk, she heard a minor kerfuffle in the hall leading from the lifts, including a very distinct voice.

He's here. She sipped her coffee again and rose, wandering out to the main office floor just in time to see Sherlock Holmes lurch into view. Draper had cuffed him and seemed to almost be shoving him along; she was barely five feet tall and Merivale, not for the first time, had to admire how scrappy she could be when the occasion called for it.

"Merivale-"

"Oi," Draper said, tugging at Sherlock's cuffed wrists a little to get his attention. "You've been arrested and you're under caution, do you get that?"

Sherlock glanced at her over his shoulder. "Yes, I get that," he said disdainfully, then turned back to Merivale. "I need to speak with you," he said. "Privately and immediately. It concerns the case, and it can't wait."

Merivale hesitated for a few seconds. Then she exhaled lightly. "Okay," she said. "Five minutes. And then you'll kindly submit to fingerprinting. You can uncuff him now, Draper. Could you go and get the procedurals sorted out?"

Draper reluctantly fumbled to unlock Sherlock's handcuffs, not bothering to help him after they'd been unlocked. Merivale beckoned one finger at Sherlock and led him into her office, where he shut the door with an emphatic clunk.

"Celeste knew something," he said. "Something about Bob Thompson. And if your constable hadn't arrested me, I think I stood a very good chance of finding out what that was."

Merivale dropped into her seat. "It might surprise you to learn that I'd already supposed something like that was going on," she said, clasping her fingers and looking up at him through her thick fringe of greying dark hair. "So what do you want me to do, exactly?"

"I want you to post a police guard of two uniformed and armed officers at the home of Greg Lestrade and every single surviving member of his team," Sherlock said. "And I want you to order it immediately. If past performance is anything to go on, this killer could strike again within hours."

"Do you have the faintest idea how the police force works, Mr. Holmes?" Merivale asked. "Officers like to get paid for their work. Which means that your suggested uniformed officers will need to be paid overtime. Which I have to apply for and receive permission for. Round-the-clock surveillance for ten detectives means another twenty uniformed officers, at the least. Just how many officers do you think we have, that we can spare twenty of them to sit about doing nothing for an indefinite period of time?"

"They won't be doing nothing," Sherlock said testily. "They will be serving and protecting a team of detectives…"

"Detectives you've yet to prove are in any significant danger."

Sherlock slammed his open palm down on the desk, close to Merivale's fingers. She looked up at him.

"Right," she said. "I've had quite enough of that from you, Mr. Holmes." She stood up and went to the door of her office. "Draper," she said. "Get Mr. Holmes fingerprinted and given a nice comfortable cell, will you?"

"What? You can't keep me here."

"Oh, I'm afraid I really can. You're under arrest for trespass, and I can hold you for twenty-four hours. That's if you really have no idea where Matthew Lestrade really is. May I remind you that he is a minor?"

"Oh, for God's sake," Sherlock groaned. "And may I remind you what happened the last time I was accused of kidnapping a child...? Oh, fine." He stood up, resigned, as Sarah Draper appeared in the doorway again. He held his hands out to her. "Do you want to cuff me again, too?"

"Don't be a smart-arse," Draper said without smiling. "Just come over here and we'll go through the procedurals. You're still under caution, too. Take the hint and shut your mouth."

It was another twenty minutes before Merivale, kneading her shut eyes with her fingertips, heard a familiar rap on her office door. She looked up. Not Draper, this time; Pinari.

"Yes?" she said, a little testily.

"Greg Lestrade's just arrived, Marm," Pinari said, ducking his head in that obsequious way that always managed to get on Merivale's nerves. "Has his partner and daughter with him."

"Good." Merivale got up, scrabbling for a pen on her desk. "I need to speak with them, too."

Greg Lestrade was, as Pinari had said, in the waiting room with the two women. Merivale paused in the doorway for a second to reflect, with a tinge of malice, that they could both be his daughters. But before she could open her mouth, he crossed the room to her.

"Hey, June," he blurted out. "I need you to run a check -"

"Yes, thanks for getting here, finally." Merivale glanced at her watch. "Are you generally three and a half hours late to work?"

Lestrade gritted his teeth for a second and swallowed down on something. "Work? Does that mean I'm getting paid for my time just now?"

"Don't do this, Greg. I don't want to end up arresting you, too."

"No, you don't – you have other things to chase up. My daughter took a call from her mobile in Bloomsbury half an hour ago. We think it's either from Matthew or from the killer. You need to do a full trace on that call. June, if a killer is calling my daughter I'm - "

"We'll get the tech team on it this afternoon, Greg – just calm down, okay? There's nothing we can do about it in the next two minutes. I can…" She paused. "I can post an officer at your house for security if you're worried about your safety," she finally said. Then, determined to be fair, "Holmes doesn't think it's a bad idea, anyway."

He blinked. "What?"

"Your sniffer dog was arrested at Severndroog Castle an hour ago," she said, trying to keep the contempt out of her voice. "He's here."

"Where?"

"Cell 2D. Greg, we're not finished – "

"Yes, we are." Lestrade brushed past her, turning a corner in the corridor behind on his way to the holding cells. Only one of them seemed to be occupied. The supervising officer, a young man he didn't recognise, stared at him in some consternation.

"Hi," he said, fishing his warrant card out of his pocket. "Detective Inspector Lestrade. I need to speak with the prisoner in Cell 2D, now."

The young constable glanced uncertainly over his shoulder as Merivale arrived. "Marm?" he asked deferentially.

Lestrade turned to her. "Two minutes," he begged. "Give me two minutes."

"I'll do you better than that," she said. "I'm not a bloody ogre, Greg. Go on."

The constable handed over his keys, and Lestrade went in, roughly elbowing the heavy door shut. Sherlock had been seated on the bench on the far wall, but on seeing who it was, he scrambled to his feet so abruptly that he nearly tripped over them. "Greg," he blurted out. "Listen to me. I need you to talk to whoever you have to to get me out of this cell immediately. Then take Melissa and Hayley home and stay there until I've figured out how to stop the killer. You-"

"Where's Matthew?"

Sherlock let out a breath and dropped his shoulders. "He's… safe," he finally said, covering his face with his hands for a second, as if aware of the inadequacy of his reply. "Did you speak with –"

"No," Lestrade said quietly. "That's not a good enough answer. Where's Matthew?"

Sherlock looked at him in silence, then shook his head.

"I can't believe you…" Lestrade took a step back. "Seriously? I asked you to help me get my son back, and now-"

"No, you didn't," Sherlock said. "You asked me to find him. I found him. You asked me to make sure he was safe, and while he stays where I told him to stay, he is safe..." Sherlock trailed off, watching as Lestrade suddenly folded forward, hands on knees, like a man who'd just run a marathon. He took a couple of breaths in and out.

"Oh, my God," he finally said. "Sherlock, that's bloody abduction, and you've just confessed to it…"

"Yes, just like I kidnapped and poisoned the Bruhl children," Sherlock snapped. "Not exactly the finest moment of your career, that little mistake, was it? You've trusted me before. Trust me now, and get me out of this cell so I can find this killer. Call Commissioner Hale if you have to."

"No. Not until you tell me where Matthew is."

"For God's sake, I don't have time for this game, and neither do you, because you're in danger. You all are. I have no idea who the murderer will target next. I won't know until I investigate more, and I can't investigate in a holding cell. Which of your team would you prefer to lose next? Do you want it to be Dyer?"

For a few seconds it was so quiet both could hear a phone ringing at the reception desk, four cells over.

"Go to hell, Sherlock," Lestrade said. Before Sherlock could process this, he turned and left, slamming the cell door shut behind him.