All was curiously still on the other side of the door. Receiving no answer to his warning, John edged it open to find Edward a few feet away, clinging to the landing railing. John knew that look immediately, and sprang forward. "Hey-"

Too late. Edward heaved, splashing vomit onto the stairs below. Caitlin, alarmed, took a step toward her brother, but Edward elbowed her out of the way and awkwardly turned himself around. He still held fast to the railing with both hands, white-knuckled. John tried to get a good look at Edward's eyes, but the overhead light was dim and Edward shut his eyes. "What are you looking at?" he demanded.

"Nothing," John said immediately, taking a step back. "How's your head?"

Before Edward could respond, Caitlin stepped forward, the roll of duct tape in one hand and the gun in the other. She hadn't time to ponder the logistics of rebinding John's wrists before her brother interjected again. "Not yet, stupid," he said. "How's he supposed to write with his hands together?"

Write?

Caitlin, levelling the gun in her right hand, fumbled for the pocket of her hoodie without taking her eyes off John. It was a few seconds before she was able to put the roll of duct tape away. "Downstairs," she ordered, bringing both hands back up to the gun. "Now. If you run, I shoot."

John didn't know whether Caitlin would really shoot someone in the back, but he wasn't willing to try her on it just then. He let her march him ahead of her down the stairs, doing his best to avoid the vomit on the carpet. "Your brother's got a concussion," he said to her over his shoulder as he reached the bottom. "He needs to be in hospital."

And now, it's just possible that I do, too.

The ground seemed to shift from underneath him like a conveyer belt, and he grasped the balustrade to steady himself for a moment before continuing down the hall and into the kitchen. Caitlin pointed to one of the kitchen chairs, and he sat down on it with as much poise as possible. His head felt dangerously heavy, lolling a little under its own weight. But he was clearly in a better state than Edward, who staggered in after them, bloodstained fingers clutching at the wall as if it could hold him up.

This might have been John's moment, if Caitlin hadn't found and confiscated the gun. But she had. He looked down at his fingers, tapping them restlessly on the tablecloth. Perhaps it was for the best that he didn't have an opportunity to take out Edward right now. He knew he had ten fingers; the only problem was, he was counting no less than sixteen. He watched them for a few seconds; they seemed to be writhing like snakes. Then Edward shoved something under his right hand. Looking down, he saw that it was the back of an envelope. Edward put a ballpoint pen down on the table with a soft plink.

"What's this?" John asked, shifting the envelope from his right hand to his left and reaching for the pen.

"I want it in writing," Edward said, "that you murdered our father."

John gave a bitter, brief little chuckle. "No," he said, and put the pen down.

"Do it." Edward lifted the crossbow in his shaking hand. "I won't miss from this close."

John looked Edward over in silence for a few seconds. He was a mess, but no, he probably wouldn't miss from point-blank range. With a light sigh, he lifted the pen and scrawled out:

To whom it may concern,

I, Dr. John Watson, confess to the murder of Jeffrey Hope as being my actions and mine alone.

"I hope that'll do," he said, signing off with a vague approximation of his signature and pushing the piece of paper back across the table. He pulled his chair out and tipped his head forward, kneading his fingers into the back of his neck. "'Cause I really don't feel like writing a novel on it. I s'pose you're going to shoot me now anyway."

Sherlock shoved his phone in his pocket with his free hand and made his way through the crowd of officers to where Molly was still sitting in the driver's seat of the car. "Sharon sent over Hope's post-mortem report," she said, handing her phone to him. "I don't know if you can read it on such a small screen…"

"Yes," Sherlock muttered, peering down at it.

"I haven't had a chance to look at it properly yet, but did you know -"

"He had an aneurysm, yes. It was part of his motive, as well as his motivation. He had nothing to lose." After reading in silence for a minute or two, Sherlock set Charlie on her mother's knee and knelt down beside her, clearing his throat. "I spoke to Mycroft," he said.

"What did he say?"

Sherlock paused. "He offered to send snipers," he said.

"Snipers? No." Molly shook her head. "It's too dangerous. Merivale said… she said, if we keep the line of communication open…"

"Merivale's working on the assumption that the Trent children intended to take John as a hostage," Sherlock said. "They didn't. They intended to get into the house, somehow - probably just by knocking on your door - kill you and Charlie, and leave before John even got home. Edward miscalculated. He sent a text to John gloating about it, probably assuming he was still at the hospital with Donovan. But he'd already left and was halfway home, which is how he was able to reach you first."

"And now they don't know what to do?"

"Precisely."

Molly shut her eyes. "But if they were going to kill me and Charlie," she said, "then why don't they just kill John?"

"I don't know," Sherlock muttered, glancing down at his shoes. "My guess is that either they're discussing whether they should take advantage of the situation and make up some demands, or they realise that if they kill him, they'll then have to kill themselves. Or face trial for murder."

"They'll have to do that anyway. They've already murdered three people."

"Yes, but this one will be an open and shut case. This is a gift, Molly. Now we just have to use it to our advantage."

Detective Inspector June Merivale was in a quandary. On the one hand, she knew that the standard response to this sort of incident was to bring in an armed tactical response team - one step below military-level snipers. On the other, she knew that bringing in officers in riot gear was possibly going to make things worse, both inside the house and out. There had been absolutely no response from the Trents, or their hostage, in half an hour. The afternoon was fading, but there seemed to be no lights on inside the house. No noise. No movement. No confirmation at all that John Watson was still alive in there. Two ambulances and two teams of paramedics had just arrived and parked, waiting to be needed.

"Right," she said to Lestrade, pulling her phone away from her ear. "I've had enough of calling the phones - pretty obvious that they're not going to answer."

Lestrade nodded, watching as Sherlock left Charlie with Molly and made his way over to them. "So what do we do now?"

"Now," she said reluctantly, "we get out the PA system and keep trying with that, I suppose. Let them know we're willing to listen to them-"

"They're not interested in talking," Sherlock interjected. "They didn't intend for this to happen. They have no demands. They don't know what they want."

Merivale glanced at Lestrade, who shrugged.

"So what do you suggest, then?" she asked Sherlock, trying to keep the ice out of her voice. "We can't just stand out here forever while they figure out what they want. And if they want to murder John..."

Sherlock took a breath. "Let me try," he said. "Let me try calling. My ID shows up on the landline and John's mobile. They may want to speak with me."

"Why?"

"Because Hope was trying to kill me when J- when he was shot," he said. "And because this is a lot less effort than setting up a PA system."

Merivale turned to Alan Peters, giving an order to have the PA system set up anyway, and asked for him to confirm that the tactical response unit were gathered in a nearby street that couldn't be seen from the second storey windows of the Watson's house. Peters nodded and went. After what seemed like a year, June turned back to Sherlock. "Sure," she said. "Try, by all means. If you think it'll help."

Sherlock pulled out his phone again - noting that it was now starting to run low on batteries. John had probably had his own phone confiscated, so he tried the landline first. As he listened to the purr of the dial tone, he watched Merivale pull out a cigarette and Lestrade light it for her, then light his own. Obviously the Metropolitan Police threw their no-smoking policy out the window when their senior detectives were under stress.

He hadn't expected anyone to pick up, but halfway through the fifth ring, there was a barely perceptible click. He pulled the phone away from his ear for a second to check the display. The line was open.

"Caitlin," he deduced. Girls were more programmed than boys toward social pleasantries like answering the phone, and he knew Edward was injured. "Sherlock Holmes. Let me speak to John."

Lestrade and Merivale looked at each other again. Then Merivale, still with her lit cigarette in hand, rushed over to the uniformed officers milling around the squad car parked across the street.

"Um, that's not how this works," Caitlin said.

"No, it isn't," Sherlock agreed, making eye contact with Lestrade for a second. "How this usually works is that we deploy flares and burn the house down while you and your brother are standing in it. You will give me proof that John Watson is still alive, or that's exactly what will happen."

He listened to Caitlin breathing on the line. In. Out. In.

"One minute," he said. "Sixty seconds. Will you give me sixty seconds?"

Then came a series of shuffling sounds and low voices, neither of which belonged to John. After such a long pause that Sherlock was convinced that Caitlin had just put the phone down and walked away, it was picked up again. "Yeah, Sherlock," John said wearily into the receiver. "Still alive. I'm fine. Is Molly okay?"

"You're fine…?"

"Oh, for God's sake… um. I'm just… Does Mycroft know about this?"

Sherlock exhaled. 'Does Mycroft know about this?' was a confirmation that John was speaking on his own terms. The code for any coercion was denial that it was taking place.

But there was something else that was worrying him. John's breathing was heavy and irregular, and he slurred his words a little.

"You're not fine," he said. "Are you injured?"

"Whack on the head," John mumbled. "Room's spinning a bit, but it's not that bad. I…" He trailed off, and Sherlock heard Caitlin mutter something in the background of the call.

"I've got to go," John said. "Tell Molly I'm fine…"

"No, don't hang up," he said. "Put Caitlin back on."

There was more fumbling with the phone and low voices. He heard Caitlin's become sharp for a second or two, though he couldn't make her words out. "Yeah," she finally said into the receiver. "What do you want now?"

"To confess."

"… What?"

"You've got the wrong person. John didn't kill your father. I did."

"Um, yeah, that's cute of you, but too bad it's bullshit."

"I have a post-mortem report here - no, you listen to me, because whatever patience I ever had toward you has just run out. I have a post-mortem report here. It was conducted by a third party not associated with me, John, or Scotland Yard. In it, it outlines that your father was suffering from a cerebral aneurysm, which was diagnosed in May of 2007 with a CTA performed at the University hospital. Believed to have been caused by atherosclerosis. Because of its size and unusual location the risk of death during surgery was so high that your father elected to not have that treatment. It would have eventually killed him."

Caitlin paused. "Right," she said. "So you gave my father atherowhatever, then?"

"The report also reveals," Sherlock went on, "that your father died as a combined result of shock and trauma to the shoulder inflicted by a blunt object after the shooting had taken place. That blunt object was my shoe, Caitlin. I needed information from him, and I extracted it. It's possible that he would have survived the initial shooting if I'd provided First Aid quickly. I didn't."

After a long silence, Sherlock pulled the mobile away from his ear and looked at the display screen. Caitlin had hung up on him. With a growl of frustration, he shoved the phone into his pocket and turned to Lestrade, who had been smoking and watching the whole thing in silence.

"We've got to get in there, quickly," he said to him, fighting off the sudden, insane urge to confiscate his cigarette. "John mentioned something about being hit in the head. He's slurring and confused. Signs of a serious concussion that might kill him if it goes untreated."

Lestrade looked at him. "Oh, shit," he said. "Sherlock, we're already going as fast as we can…"

"And that's not quick enough," Sherlock said. "I've told Merivale she's analysing the situation incorrectly, and she won't listen to me. Will you listen to me?"

After a barely perceptible pause, Lestrade nodded. "Okay," he said. "Okay. What do you need?"

"I need to get in there. Now."

Even in his light-headed state, John wasn't the least surprised when Caitlin sat down at the table opposite him and reached into her pocket. He'd seen at first glance that it was bulging. She brought out two amber-coloured bottles with white childproof caps. Generic bottles, the sort medical labs kept on hand for miscellaneous projects. He swallowed down a wave of violent nausea.

"I think," he said, "I know what you're about to suggest."

"It's not a suggestion," she said. "'Cause you don't have a choice, except if you'd prefer a bullet in your head, like the one you gave my father."

"I didn't hit him in the head," John muttered. "If I'd been aiming for his head, I wouldn't have missed… no." He shook his head, promptly regretting it as the room spun. He took a deep breath. "You've got something up your sleeve," he went on, articulating his words carefully. "A cheat or an antidote. This might've worked on Sherlock and it might've worked on some terrified hostages, but it's not going to work on me. Forget it."

Caitlin raised the gun.

"So this is how you were going to kill my wife, then?" John raised one eyebrow. "You were going to force her to take poison at gunpoint. Well, at crossbow-point, anyway. That's really brave - two of you against an unarmed woman."

"Same odds that my dad had," Edward said. John glanced over at where he was sitting on the sofa, still cradling his precious crossbow. It wouldn't be difficult to operate. He could probably still do it, even if he was a mess of concussion by this time.

John took a deep breath. Concentrate. You need to concentrate.

"You were the one who killed Celeste, weren't you?" he said to Caitlin, completely ignoring her brother. "Dyer said it… said no way she'd accept a drink from a teenage boy. She wasn't that stupid. But she thought you were her friend… told you all about Matthew's book. It's how you found out about my blog. What a thing to do to your best friend…"

He stopped, suddenly remembering Dyer's claim that Caitlin had done a lot of blubbering about Celeste being her best friend. John hadn't been at the initial interview at the Trent's house, but was happy to place bets that Caitlin hadn't mentioned their being besties that day. And then, the incident with the snake. Help her. Caitlin was a pretty enough girl, in her own colourless, ironed sort of way. But at night, wearing no makeup, and with a hoodie covering her hair, she could easily pass for, say, a young man.

"'Help her'." He smiled to himself, then shut his eyes and tilted his head back slightly, wincing as a bolt of pain shot down his neck. "We all thought that might've been about Jones, but she was already dead… that was about Donovan. You could've been a bit more plain about it. 'Help Donovan 'cause my brother's about to kill her. Love, Caitlin Trent.' Would've been nice. Why try to save Donovan, though, of all people?"

Caitlin looked sulky.

"You liked Donovan?" he pressed, swallowing another wave of nausea. Concentrate. Don't lose this. "You weren't even there when Jones was killed, were you? You threw up when your brother roped you into helping kill Thompson, and after you killed Celeste, you wanted out-"

"Shut up and pick which pill you want."

"I don't want either of them," he muttered. He glanced over both bottles but, realising they were virtually identical, looked up at Caitlin instead. Reading her expression and body language would have been so much easier, had he not been seeing two of her. He glanced over at Edward again.

"There's something you mightn't know," he said to Caitlin, leaning forward across the table in an avuncular sort of way and lowering his voice so that Edward couldn't overhear. "After… after the night your Dad died, the police took both pills and tested them. They were both poisoned, Caitlin. It's right there in the report - I'm sure Merivale'd let you have a look at it. Your Dad had been working for Jim Moriarty. When people stopped being useful to him, Moriarty killed them." He looked at Edward again. "You ever wonder if your brother might be a bit like that? Was it his idea for you to play this game with me?"

Caitlin swallowed. "Five seconds," she said.

"Don't you think he should have offered to play the game with me himself, if it was his idea all along?"

"Four."

"Wait, hang on," John protested. "I don't think any version of this game ever had a time limit like that-"

"Three…"

With a sigh, John swiped up the bottle in front of Caitlin, twisted the cap off, and tipped the capsule into the palm of his hand. Innocuous, ordinary looking thing, half clear, half blue, and the contents looked to the untrained eye to be nothing more sinister than grains of sugar.

"That's your choice?" Caitlin's wide-set, brown eyes gave nothing away, and John wondered for a second if she even knew which pill was which. He nodded, watching her as she tipped the other capsule into the palm of her hand.

"Count of three?" he suggested.

He half expected some attempt at sleight-of-hand at the last second. But as he worked the capsule down, he noted that Caitlin had some honour, at least - she'd also taken hers. He had no idea what her pill was like, but his seared the whole way down, and he was half-tempted to ask for water. Caitlin gagged and swallowed twice.

From the sofa, Edward chuckled softly.

"So," John said, splaying his hands on the table-top and ignoring Edward. "Out with it. Which one of us got the poisoned pill?"

Caitlin smiled. "You did."