We take a quick diversion to send some of our homeless network looking around and text a few others before heading off to Scotland Yard.

Lestrade meets us by the door of his department's office and hands dad a sheet of paper as we walk inside.

"This fax arrived an hour ago," he says and dad flips it over so we can read the print. A large handwritten note on the paper reads:

HURRY UP
THEY'RE
DYING!

Dad and I exchange glances before he passes the note over to John.

"What have you got for us?"

"Need to find a place in the city where all five of these things intersect," dad replies, handing the piece of paper we were using in the lap to Lestrade.

"Chalk, asphalt, brick dust, vegetation ..." he reads. "What the hell is this? Chocolate?"

"I think we're looking for a disused sweet factory," I say.

"We need to narrow that down," Lestrade groans. "A sweet factory with asphalt?"

"No. No-no-no," dad disagrees. "Too general. Need something more specific."

"Chalk," I suggest, "chalky clay – that's a far thinner band of geology." Dad nods and I visualise a map of London and mark out the areas fitting that terrain type.

"Brick dust?" Lestrade asks dad who has his eyes closed, clearly doing the same as me.

"Building site. Bricks from the 1950s," dad replies and Lestrade rubs his face in despair.

"There's thousands of building sites in London."

"I've got people out looking," dad says, exasperated at the continued interruptions.

"So have I," Lestrade says defensively.

"Homeless network – faster than the police," dad says, smiling snidely. "Far more relaxed about taking bribes."

I notice Anderson listening at a nearby desk and he rolls his eyes at this. I shoot him a glare as dad's phone starts to trill a series of text alerts. He brandishes the phone triumphantly at Lestrade and I come over to look at the pictures.

He sifts through each of them before stopping at a picture of a close-up of some purple flowers.

"John," dad says, holding the phone out to show him the picture. "Rhododendron ponticum. It matches." I know nothing about geography and botany so I'm at a loss here. "Addlestone," he says a moment later.

"What?" Lestrade asks.

"There's a mile of disused factories between the river and the park. It matches everything." He turns and hurries out the office and John and I follow quickly behind him.

We race over to Addlestone with several police cars following and all of us head inside the large factory.

We all switch on our torches as we are plunged into darkness as Donovan begins to coordinate the police officers.

"You, look over there," she starts. "Look everywhere. Okay, spread out, please. Spread out."

Lestrade leads us and his team towards another part of the factory.

"Look in there," he says softly. "Quietly. Quietly."

As we head deeper into the factory, we reach a large, clear area littered with a number of empty sweet wrappers and a candle. Dad walks forward and touches the wick of the candle before drawing his hand quickly back.

"This was alight moments ago," he says before calling out loudly. "They're still here. Sweet wrappers," he says, dropping his voice again. "What's he been feeding you?" Dad picks up one of the wrappers and examines it more closely. "Hansel and Gretel." He holds the wrapper closer to the beam of his torch and sniffs the paper before touching it with the tip of his tongue and grimacing. He looks at the wrapper in startled realisation. "Mercury."

"What?"

"The papers: they're painted with mercury." John groans in response and dad continues. "Lethal. The more of the stuff they ate ..."

"It was killing them," John says.

"But it's not enough to kill them on its own," I say, frowning.

"Taken in large enough quantities, eventually it wouldkill them," dad explains. "He didn't need to be there for the execution. Murder by remote control. He could be a thousand miles away. The hungrier they got, the more they ate ... the faster they died." Dad grins. "Neat."

"Sherlock," John warns reprovingly.

"Over here!" Donovan calls out and we run in the direction of her voice. "I've got you," she says softly to the huddled children. "Don't worry."

The girl is taken to Lestrade's office for questioning and we have to wait outside while Lestrade and Donovan go in.

Dad gets restless and begins pacing around outside for ten minutes before they come out.

"Right, then," she says sarcastically with a hint of bitterness. "The professionals have finished. If the amateurs wanna go in and have their turn ..."

"Now, remember," Lestrade warns, giving dad a serious look, "she's in shock and she's just seven years old, so anything you can do to ..."

"... not be myself."

"Yeah," he agrees. "Might be helpful."

Dad looks around to me and just about holds back an eye roll as he folds down his coat collar. I lead them inside where the girl is sitting at the table, looking down as the female liaison officer strokes her arm reassuringly.

"Claudette, I ..." Dad starts but gets no further because the girl lifts her head, takes one look at him and begins to scream in terror. My eyes widen in confusion and look at dad. "No-no, I know it's been hard for you ..." She continues screaming and scrambles to get away while pointing at him. "Claudette, listen to me ..."

"Out," Lestrade shouts. "Get out!" He grabs dad's arm and bundles him out the room as the girl continues to scream. I follow behind him and we head away from the office to reconvene.

"Makes no sense," I say, shaking my head.

"The kid's traumatised," Lestrade explains. "Something about Sherlock reminds her of the kidnapper."

"So what's she said?"

"Hasn't uttered another syllable," Donovan replies.

"And the boy?"

"No, he's unconscious; still in intensive care."

Flickering lights in the building opposite draw my attention as all of the lights come on at once, revealing three spray painted letters painted onto the windows.

I O U

The lights go out on that floor seconds later and dad and I exchange uneasy glances.

"Well, don't let it get to you," Lestrade says to dad, trying to lighten the mood. "I always feel like screaming when you walk into a room! In fact, so do most people. Come on." He turns and heads out, followed out by John. We go to follow him but Donovan stays behind for a moment.

"Brilliant work you did, finding those kids from just a footprint. It's really amazing," she says.

"Thank you," dad replies but Donovan continues pointedly.

"Unbelievable."

Dad hesitates momentarily before we continue on, heading outside to meet John.

"Ah," he says as we reach him, raising a hand to hail an approaching taxi. "You okay?"

"Thinking," dad dismisses as the taxi pulls up at the kerb. "This is my cab. You two get the next one."

"Why?" I ask.

"You might talk." I raise my hands in protest but he gets in and closes the door and the taxi pulls away. John stares after him in disbelief, then sighs.