"Molly…"

Molly half-opened her eyes, then sank back down into near-oblivion again. So tired…

"Molly! Get up!"

Her eyes flew open. She found herself curled up awkwardly on the living-room sofa. The TV was on, and John was standing over her, still shaking her by one shoulder.

"Get up," he barked. "Jesus, don't you ever answer the phone?"

Her phone… was her phone even downstairs? Or had she left it in the bedroom…? She hadn't the faintest clue what the time even was. She hadn't meant to fall asleep on the sofa. She'd put Charlie down for her nap, and then sat down to watch the episode of Hannibal that she'd taped while they'd been at Greg's the evening before… only closed her eyes for a minute…

"Come on, Molly! We're leaving. Now!"

Her heart gave a painful thump. Married life wasn't always a romp through a field of daisies, but she didn't think John had ever used that tone of voice with her before. Before she was properly on her feet, he'd already rushed out of the room. She went to the kitchen and scooped up her handbag from where it sat on the counter, listening to John's heavy footfalls on the stairs and then in the nursery above her head. Then back to their own bedroom, and she heard the shuffle of the bedside drawer being pulled open. He was fetching the gun.

She'd just reached the front door, and was fumbling with her shoes, when he reappeared at the foot of the stairs with Charlie, sleep-flushed, in his arms. Her backpack of various paraphernalia was hanging off his wrist.

"John," she said. "For God's sake, tell me what's wrong!"

"I'll tell you in the car. Please, move."

"But… do I have time to pack anything?"

"No. I'll come back and get stuff later. Let's go."

The sun was now low in the sky, and the shadows were long. Looking around, the street seemed to be deserted, except for a grey-haired woman watering her garden in a house up near the corner. But they had just made it to the car, and John was fumbling in his pocket for the key, when Molly saw a movement in the shadow of the hedge next door. Two figures stepped out from behind it.

John's breath caught. She felt, rather than saw, his shoulders drop.

Both were strangers to Molly. A blonde girl, twenty at most. A boy with blood clumping his dark hair and splattered all over his t-shirt and the knees of his jeans. He held up something like a huge, black insect, all spindly legs and wings. In another instant, she saw that it was a small handheld crossbow, loaded with a bolt the thickness of a man's thumb.

"Molly," John said without looking at her. "Don't."

White-hot anger flooded over her.

This was the moment they'd all both been anticipating for a year. The reason John had trained her for months on how to handle his gun - so she could use it if some psychopath aimed a crossbow at him and Charlie. And now he was telling her not to take it, not to shoot? She forced herself not to glance at where she knew he'd tucked it into his belt, just near his left-hand jeans pocket. If he didn't want her to use the gun, he certainly didn't want her to give away where it was.

"This is my daughter, Charlie," he said, addressing the boy. The girl he ignored. "She's thirteen months old. And I'm not using her as a human shield. So let me tell you what's going to happen. I'm going to give her to her mother, we're going to put her in the car, and then they're going to drive away."

The girl rolled her eyes. "I don't think you've, like, really grasped the idea here?"

Oh, my God, Molly thought to herself. She's practically a child.

A child who'd already killed three people.

"Oh, I think I've grasped the idea pretty well," John said, smiling grimly. He put Charlie into Molly's arms, then backed up a few paces. The boy looked confused for a second, unsure of which of them to aim at. "You two are in a bit of an awkward position," John continued. "You didn't think I'd be home in time when you sent that text, did you? And now you're stuck with one more hostage than you wanted. So I'm giving you a chance to send two of them away. It's not like they could give you any information about the night your father died, anyway. I didn't even know Molly back then."

"When he died?" the girl ground out. She reached out as if to take the crossbow off the boy, who petulantly yanked it away from her. "You mean, when you murdered him?"

"I really don't think you should be on about killing people-" John stopped himself and took a breath. "Send these two away and we'll talk," he said. "And that's my final offer. The police have been called, and they'll be here any minute. Make up your mind, before the tactical response unit shows up and decides to shoot both of you."

"What if I just shoot you both?" The boy hoisted the crossbow again, and out of the corner of her eye, Molly saw a movement from John that might have been a flinch. But was it something else…?

John now shook his head. "Bad idea," he said. He pointed to the weapon, which the boy now lowered a little, as if it were too heavy for him. "If that's what I think it is, you have to manually reload it. And while you're busy doing that, either myself or my wife are going to kill you. Both of you."

The boy seemed to waver for a second.

"Put it this way," John said. "If you let my wife and child leave, I'll let the police sort you out. If you don't, you'll be dealing with me. I really think your dad should've taken his chances with the police. So what are you going to do? Get all this way to die for nothing?" He chuckled. "That isn't very clever."

Molly listened to her heart beating for a few seconds.

"One minute," the boy said. "And I'm counting."

"You're counting. Marvellous. You passed primary school." John, fumbling in his pocket for the keys, went over and unlocked the car door. "Come on, Molly," he said.

Molly, with a glance at the boy still holding the crossbow, stepped forward. She wanted to grab John with both arms and draw him in; shove him in the car and drive away before anyone else had a chance to react.

But the last thing John needed was for her to lose her cool. As he reached across to make sure the safety harness of Charlie's car seat was attached properly, she could see that his shirt was clinging to him, soaked with sweat. "Listen," he whispered, so quietly that she had to read his lips to help her understand him. "Go to Sherlock. He knows."

"No. I want to stay with you," she said. "This is where I need to be. Here."

"Where does Charlie need to be?" Charlie had crammed John's index and middle fingers into her mouth and was chewing on them furiously with all four of her front teeth. "And I think we've got a couple more kids who can't go far without your help either, Lolly."

This posed her. "I love you," she said, and he reached out and squeezed her hand.

"Hey," he said. "They're not going to kill me."

"How do you know?"

"Because I know something they don't. I'll see you soon, okay? Drive safely."

It took four attempts for Molly to slot the keys into the ignition. As she coasted the car slowly away from the house, she glanced in the rear-vision mirror and saw John unfold his arms and wave to her.

Two squad cars, lights flashing, were blocking off the road as Sherlock's cab approached the corner. Scanning the scene, he saw Merivale and three officers crowded around a third car, a blue sedan. It was not so much "parked" as stopped in the middle of the street, as if it had broken down. Throwing a handful of banknotes at the driver, Sherlock flew out of the cab and over to the scene. A constable in uniform tried to stop him breaching the police tape barrier.

"Sir, please step away-"

"Let me through," he said. "Now!"

The constable, fingers still digging into Sherlock's chest, began a recital that the street had been blocked off and evacuated due to an "incident" and not even residents were being allowed through. This might have turned ugly, had Merivale not heard Sherlock and rushed over to pull the police tape up for him. But Sherlock wasn't paying much attention to Merivale. His gaze was now fixed on the car - the Watsons' car. The driver's door was open, and one of the PCs moved aside slightly and he glimpsed Molly still sitting in the driver's seat, with Charlie in her lap. Another PC made a move as if to take Charlie out of her arms. Molly cried, "Stop it! She wants to stay with me!" and Charlie, alarmed, burst into tears. Sherlock charged over.

"Sherlock," Molly blurted out. She was clutching Charlie so tightly the little girl was squirming.

"Did they hurt you?" Sherlock dropped on his knees beside her and gave her shoulder a little shake. "Molly - are you hurt?"

"No… no, I'm fine. We're fine. Sherlock, John practically told them about the gun. He said if they tried to shoot either one of us, the other… the…"

"Wait, what?" Merivale interrupted, giving the PC who had tried to confiscate Charlie a little push as if to tell him to go away. "Molly, are you telling us John has a gun in there?"

Molly nodded. "It was hidden in his belt," she said, "but if he told them-"

"Then he obviously did it for a reason," Sherlock finished for her, with a deprecating glance up at Merivale. "Oh, for God's sake, don't look like that," he said to her. "John's owned that gun for the better part of ten years. Licensed and registered."

"Are you sure about that?"

"If you doubt my word, get my brother's," Sherlock said tersely. "Oh. Lestrade's on his way - yes, surprise, we lied about that, moving on."

"Yeah, I know," Merivale said. "He called me just before you arrived. One hell of a phone call, that was, from a guy who died this morning. He explained who the kids are, and what John has to do with all of this."

Sherlock, getting back to his feet, looked across the car roof at the Watson's house. At a distance, and an awkward angle, it was hard to see anything. "And they're in there. Both in there?"

"Yes."

"And armed, I suppose."

"Molly says Edward Trent, or whoever he really is, has a small crossbow. She's not sure if Caitlin's armed or not. Let's hope for John's sake that she doesn't find his gun."

"I knew her," Sherlock growled to himself, bowing his forehead against the top of the car door for a second. "I knew her. The very first thing I said when we went to interview her at the house: I know you. Of course, she was probably keeping an eye on the house that day at John's, trying to work out if she'd found the right address."

There was something else that Sherlock suddenly remembered about the day she and John had seen Caitlin in the driveway of the Watson house. Caitlin had just stared while John had been reading her the riot act about her own personal safety. She never made any attempt to defend herself, or even apologise. In the end, she had just walked away, leaving John, furious, to sputter at Sherlock: Who walks straight out behind a car like that without looking?

Who, indeed? Someone who wants to create a scene. Somebody who wants to be remembered as the idiot who walked out behind a reversing car without looking, not remembered as the smiling child in a seven-year-old photograph, glimpsed once in a dark cab.

Caitlin's cover had been histrionics, double-dealing and making herself as conspicuous as possible. Edward's had been to tow the line and fade into the wallpaper as best he could.

Which of the two was the ringleader, he'd yet to decide. But his money was on Edward Trent.

He seemed about to say something else when he heard a car door slam and, looking across, saw Greg getting out of his car, parked a little further down the cross-street. There was a little murmur from the group of officers, who parted before him like the Red Sea. Nobody tried to stop him from ducking under the police tape. Merivale greeted him with a curt nod, though she waited a few seconds for him to put his hand on Molly's shoulder in silent comfort before starting with business. "All quiet so far," she said. "They've gone inside and shut the curtains, so we can't see them moving around. It's getting dark, though, so hopefully they'll put the lights on and it'll help track what's going on. We've asked for thermal imaging cameras, but it'll probably be a week before we can get them out here, and I'm hoping to get this resolved peacefully well before nightfall."

"That's… optimistic…" Lestrade glanced down at Molly and cleared his throat. "Have you called them?" he asked Merivale.

"Not yet. I need more information," Merivale said, pulling out her mobile phone, though she seemed to be navigating the browser and not the keypad. "And the first bit I need is this. Did John actually kill Hope? No - I have to know. It's important."

Sherlock and Greg looked at each other.

"I was standing next to Hope when he was gunned down," Sherlock said. "I saw nobody in the building opposite, neither before nor after the shooting."

Merivale shook her head. "Don't think I don't know what guys tell each other when they're best friends," she said. "He never even hinted to you that he was responsible for the shooting? Because I had a look at his blog on the way here, and it's pretty incriminating."

Sherlock heaved a sigh, rolled his eyes, and said, "At the time Jeff Hope was killed, John was a recently-returned war veteran with both a real and a psychosomatic injury and was in intensive therapy after being diagnosed with PTSD. His blog was suggested to him by his therapist, Ella Thompson, as a coping mechanism for returning to civilian life. He was socially isolated and in a bad financial situation, both factors when people consciously or unconsciously insert themselves into a situation they perceive as being exciting, dangerous or dramatic. He's a romantic and a drama queen, with a very strong sense of social justice. Detective Inspector Merivale, I think you're intelligent enough to draw your own conclusions from that." He took the phone out of her hands and looked at it in silence, then handed it back. "You'll notice," he said, "John begins the narrative by telling his readership he's recalling information I'd told him after, which I did."

"But then later," she countered, "he stops the 'Sherlock said' stuff and starts talking as if he was there."

"Yes. Like I said, it's common among war veterans and others with PTSD to imagine themselves into a situation, or imagine heightened involvement in a situation. Speak to his therapist. She's easy to find, and all too willing to cooperate with a higher authority when it comes to compromising her patients' privacy."

"What happened to the investigation into Hope's murder?" she asked Lestrade. "That was your case, wasn't it?"

He nodded. "It was scaled back after six months and then went to Cold Cases. We've had someone open it twice since, but there's been no new information."

"Was John looked at as a suspect?"

"Yeah, but no more than anyone else. There was no real reason to think he was responsible. Still isn't."

"His blog - "

"June, some dodgy wording on the blog of a guy recovering from PTSD doesn't constitute a reason to charge someone. You know that. We interviewed him the afternoon following the shooting. His account was that after he got to the college, he waited outside for us. We were there ten minutes later, and by that time, the cabbie was dead. John was outside, exactly like he said. His account checked out with the known facts, so I had no grounds to make John a serious suspect in the investigation. The end."

"His firearm -"

"Oh, for God's sake!" Molly snapped, causing Charlie to squawk in alarm again. "I don't care if John shot a serial killer or not. He's in there with people who've killed three times! Get him out. Now!"

"Molly," June said gently. "I'm one of about three people in London trained as a crisis negotiator. We are getting him out. But it's going to take time and information. I need to know everything possible about why these kids have taken John hostage in the first place, and make sure I don't upset them any further or make them panic. Criminals who panic do stupid things. I'm working on it. We all are."

Molly put her face in her hands. "I keep waiting," she moaned. "I keep waiting to hear the gunshot…"

Sherlock opened his mouth to tell her that John wouldn't shoot unless he had no other choice, and it was likely he had something better up his sleeve than gunning down two people who were barely out of childhood. Then he realised what she'd just said.

Gunshot. Singular.

"Where's Melissa?" he asked Greg.

"At home. Do you want her here?"

"No, I want her at home. Molly, take Charlie back to the Lestrade's."

She shook her head. "No," she said. "I left John because it was the only way to get Charlie out. I'm not leaving him to go to the other side of London! Someone do something…"

"Oh, Molly, listen to me carefully." Sherlock dropped back down beside her, taking her shaking hands in his. "Look at me and listen. I am doing something. This is how I solve crimes. I think. If we're going to get John out of there - and we will - you're going to have to stop panicking and help me think."