The girl on the floor goes still. Graves looks at her down the barrel of the pistol. She doesn't make a sound. He's aware of footsteps approaching from the north, recognises the tread, thinks: Sawyer. A glance up shows the big Texan quickly approaching, holding a green backpack in one hand.

"Get up." Graves grabs the girl by the shoulder and hauls her to her feet. He notices she watches his eyes, not the gun. She can't be more than fifteen, he thinks. He looks around him – they're off the path, in a quiet section of the park, and it's dark, but not that dark. They'd blown past twenty people, easy, on the way; knots of surprised tourists peering through the gloom in their direction. Parents, kids. People with cellphones.

Sawyer's by his side, breathing fast. Graves points in the direction of the van. "We're going. Now." To the girl: "Scream, cry out, or try to run, and I'll shoot you." Her eyes flick to the gun, but she doesn't say anything.

Musical notes from the turf underfoot. He looks down and sees the girl's own phone. Graves scoops it up and reads the screen. It's a withheld number. He gestures again to the van and Sawyer takes the girl by the arm and marches her towards it. Graves looks up the hill again and sees a man in a brown jacket walking towards them, jogging now. Definitely time to go. He turns and follows Sawyer.

The phone's still ringing; he answers and a man's voice says, "Emergency operator-"

He stabs at the END CALL button, pops the battery out of the phone, and tosses both pieces away. Graves breaks into a run and claps Sawyer on the back as he passes him. "Go! Go!"

Brody sees them coming and turns the engine on, headlights dazzling Graves as he hurdles the ankle-high wall that surrounds the parking lot. He comes round the side, the door slides open, and he piles into the back of the van. Sawyer appears, tosses the girl in like a sack of mail, and then Raghuveer is closing the door behind him.

They're out of the lot thirty seconds later. The clock in Graves's head says three minutes since the call was made. Ninety seconds since the callback. Police response, five minutes at the outside.

They could have the van's tags, he thinks. The man in the brown jacket had been close enough; close enough to see Sawyer manhandling the girl, close enough to read the license plate. It's going to go badly if the cops stop them.

The girl is propped up against the wall on one side of the empty space in the back of the vehicle. Sawyer's kneeling beside her, going through the backpack. She turns her head, looks Graves in the eye and says, "Are you going to kill me like you killed Danny?"

Ah, Graves thinks. It's like that.

He pops the clip out of the Sig Sauer and puts the gun and magazine in his coat pocket. "No," he says. "Not if you tell me what I want to know."

Sawyer brings the plastic cuffs out of the bag, looks at him. Graves shakes his head. "You can put those away, too. Check her pockets – left side of her jacket, I think. Hill passed her something."

Sawyer checks. The girl doesn't break Graves's stare. The Texan comes up with a USB stick. "What's that?" Graves asks.

There's a beat before she answers – something going on in her eyes. She says, "Data. Backups of your computers. Everything you've got."

"How?"

"Hacked the BIOS. Your laptops are running my firmware." Her voice is weary, defeated, but his eyes widen. How long have they been watching us? And how?

He looks up. The van's passed out of the park, out on to the city streets. Brody is in evasion mode, taking them on an aimless zig-zag route, no destination except away. He raises his voice, yells "Brody! Pull over!"

He gives himself one minute, no more. They're parked in the shadow of an overpass, cars passing them as he checks under the chassis with a penlight, the fender, the wheel wells… He finds nothing. He stands himself up in the side doorway of the van to look on the roof, and there it is. A rectangle of plastic the size of a paperback book. Son of a bitch!

Sawyer gives him a boost. He tears the GPS package off the roof and hurls it into the bushes at the side of the road.

He gets back in to the back of the van. Brody hits the gas. Somewhere, in the distance, he can hear sirens.

"We're pulling out," he says. He looks at the girl. "You're coming with us. Brody, take us to the airfield. We're hitching a ride. Rags, get up front, watch the mirrors. We play it cool unless we spot a tail." The Indian nods, clambers over into the passenger seat, and Graves settles down next to the girl.

He says, "You've been watching us,"

"Yes."

"Why?"

She says nothing. Graves snaps his fingers in front of her face. "Talk. Talk fast. Time is short, and if you don't start telling me what I want to know, I have no further use for you. Tell me why you've been surveilling us."

She looks away, stares into the darkness at the corner of the van. "You killed my friend. I'm avenging him."

"I know about vengeance," says Graves. "You walk up to a guy in the street and you shoot him in the head. Or you go to the cops. What the hell did you think you were doing?"

The girl gives a short and bitter laugh. "I don't have a gun. You've got the police in your pocket. What am I supposed to do? I found out who you were, and I got inside your crew, and I looked for ways to hurt you."

Graves grins at her. "Find anything?"

She points at the USB stick in Sawyer's fist. "I was hoping I might. Like, the key to the data Danny stole. It's password-locked. But I thought if I gave you some fake data, you might try decrypting it, and I could get the key that way."

"Ah: the stick you gave to Drake, in Chinatown. Everything went to the States for analysis." He shakes his head. "We never had the key. Security, you understand."

The girl closes her eyes, breathes out a long sigh.

"What do I call you, anyway?" Graves asks.

Her eyes still closed, she says "Molly. I'm called Molly."

She's silent all the way out of London, and Graves decides to let her be for the time being. Plenty of time to interrogate her once they're safely out of the country. He gets his phone out and calls Murray.

"Sir – are you still on schedule to leave tonight?"

It annoys him having to call Murray 'Sir'. The man is a billionaire's brat, a spoiled kid in a five thousand dollar suit who talks in business school slogans, not a soldier. "We're still on schedule, Lionel," says Murray, who likes to use first names as often as possible, because he thinks he's good with people.

Graves can tell he's going to start whining about being kept out of the loop and cuts his boss off. "Sir, we are inbound with five extra passengers. ETA twenty minutes. Please make all necessary arrangements for our arrival." There's spluttering from the other end of the line. "Sir, it's imperative we come in now. Quietly. It's mission-critical."

Graves knows Murray likes the military jargon almost as much as his own. He calms down. "Good. That's good, Lionel. I'll have Ruth send instructions sent to your email."

Graves cuts the call off, tosses the phone to Raghuveer in the front of the van. "Check my email and guide Brody in," he tells him.

He sits back down, leans his head back against the cool metal wall behind him, and closes his eyes, like the girl. Over the steady roar of the engine he can hear Raghuveer muttering to the driver, the fingers of Sawyer's left hand drumming on the floor, the wedding ring ticking out every fourth beat. Molly's breathing is smooth and even, like she's falling asleep.

Into the darkness Graves says calmly, "We're taking you out of the country. You're going to disappear for a while. It's up to you how long, and how much is left of you when you come back." He lets the words hang in the air, hears her breath catch in her throat.

"If you come back."

The next instalment of Root will be available on Tuesday. If you can't wait till then, take the Acenet challenge to see if you have what it takes to join this secret world. Then join the discussion on our Facebook page and test your wits against the top Acenet members