Richard greets us, and introduces his companions as Tolya and Gennadi. "All set?" he asks, and Charlie and I indicate that we are. Dry-mouthed with apprehension, I follow them all to the end of the corridor, where Richard inputs the door exit code, and summons the lift. We descend in silence to a basement, two floors below ground level, and step out into cold darkness. Richard touches a switch, illuminating a dusty, damp-smelling Aladdin's cave of packing cases, electrical generator components, construction materials, ladders, rusting fridges and traveling trunks, among which I can distinctly hear the scurrying of rats.

They come for us in late afternoon, when the light is beginning to fail. There's Richard, incongruously dressed in a Russian army greatcoat, a hard-eyed young guy with a submachine gun slung over his leather jacket, and an older man in a crumpled coat carrying what looks like a miner's helmet.

We follow Richard through this detritus to a steel door set into a central column. He tilts his head toward an overhead camera, waits for the facial-recognition software to execute and pushes the heavy door open. Ahead of us an iron spiral staircase descends into darkness. A click, and a succession of fluorescent lamps flickers into life. Richard and Gennadi lead, Charlie and I follow, Tolya brings up the rear. There's an icy, sulfurous updraft which grows stronger the further we descend, and I'm glad of my thermals and cold-weather gear.

"What is this place?" I whisper.

"People call it the reverse world," Richard says. "That water you can hear is the Neglinka river, diverted underground in the eighteenth century. There's a whole network of tunnels, sewers and watercourses down here. In the old days, there were also KGB listening posts. Gennadi used to work in one. He's one of the few remaining kroty, moles, who know their way around the network."

Finally we reach a concrete floor. Richard takes a torch from his pocket, and Gennadi puts on his helmet and switches on the headlight. We follow them into a dark tunnel, the lights illuminating weeping brick walls and an iron walkway. From beneath the walkway comes the sound of rushing water. It's rank-smelling and creepy as fuck.

"You could get lost down here and no one would ever find you," Gennadi tells me, his headlight beam sweeping across a crop of grayish mushrooms growing from the brickwork. "I've seen skeletons down here. Most of them from Stalin's time. You can tell by the holes in the back of the skulls."

"Jesus never came down here," Gennadi says grimly.

"Jesus."

"Who are they?" I ask Gennadi and he shrugs.

"Addicts, ex-convicts, hermits… Some of them live down here for months at a time."

There are about twenty of them in the group. Pale, ageless figures, dressed in threadbare uniforms and coats, who stare at us incuriously as we approach. One, a thin young woman with pinched features, points a finger at me accusingly, her mouth working in silent anger. I'm shaken to see people living in a place like this but Charlie seems unfazed. Perhaps if you've done time in a prison like Butyrka nothing ever seems weird again.

The brick tunnel comes to an abrupt end and we step out into a chamber supported by arches of discolored brick and lit by strings of low-wattage electric bulbs. Iron walkways run the length of the chamber and bridge a deep channel through which river water is flowing. With a shock I see men and women moving in the dark shadows by the walls.

We follow the beam of Gennadi's headlamp along the narrow pathway beside the river channel. Shining stalactites hang from the vaulted brick ceiling. At intervals water-drops fall from these to the river surface, the percussive sound echoing in the silence. We continue for ten minutes, perhaps more, and I become aware of a distant rushing sound. This gradually builds in volume until we reach a weir, where the river cascades over the lip of the channel into a pool some five meters below.

Handing Gennadi his torch, he begins to descend a steel ladder affixed to the vertical face of the ledge on which we're standing. At any other time, the sight of a senior MI6 officer in an overcoat and tie climbing into an underground river would be noteworthy, but I have seen so much that is terrifying and strange in recent days that I barely give it a thought. And then Richard appears to vanish.

I stare at Gennadi and he grins. "You go next. You'll see."

Nervously, I begin a torchlit descent of the cold, wet rungs. Beneath me, in the darkness, the river churns and roars. Then Gennadi angles the torch beam behind the waterfall, and I see that there's a gap just wide enough to slip through. Beyond it, just visible in the wavering beam, is the interior of yet another tunnel. Richard steps into view, and extends an arm. I take it, and as I half-step, half-leap toward the tunnel, he hauls me inside.

"I'll go first," Richard says. "I've done this before."

"OK, difficult bit," Gennadi says. "Tunnel is behind waterfall."

"Fuck," I gasp.

"All right?" Richard asks.

"Just about."

When the others have crossed safely, Richard turns to a door facing us a short distance up the tunnel. This is protected by a number code, which he taps in, masking the keypad with his body. As the door swings open, he and Gennadi shake hands. "Go safely," says the mole, raising a hand to us before stepping back behind the waterfall. Soon the beam from his helmet is no longer visible.

A pale light, however, shines from behind the half-open door. We're on a walkway near the top of a huge cylindrical shaft. Below us, staircases descend in a series of zigzags for at least a hundred meters. Richard loses no time, beckoning us to follow him. We descend the stairways at speed, passing floor after floor, our boots thudding on the metal treads. The deeper we go, the eerier the place looks. The steel-ribbed walls are coated with flaking red anti-rust paint, while the fittings look decades old. Scuffed dust and flattened cigarette ends suggest that others have used these stairs recently, and after a time a faint hum becomes audible from below. It takes us about ten minutes to reach the bottom of the stairs and a makeshift atrium where an armed guard waits for us, the winged shield on his uniform identifying him as an officer of GUSP, the former 15th Directorate of the KGB. The espionage nerd in me can't help being a tiny bit thrilled at this. In London, we knew GUSP as the most secretive of the Russian security services. We had no idea what they actually did.

Richard shows identification and the officer nods us past. An automatic door opens in front of us, the sulfurous smell is suddenly stronger, and we follow a corridor into a scene so unreal that Charlie and I both stop dead. We are on the deserted platform of an underground railway station. Both to the left and the right the track vanishes into unlit tunnels. Opposite us, on a wall faced with glazed tiles, is a bronze hammer and sickle a meter high and an enamel sign reading D6-EFREMOVA.

"What is this?" I ask Richard.

"Efremova station," he answers. "Part of the D-6 underground network. Officially, D-6 doesn't exist. Unofficially, it was built by Stalin to link the Kremlin to underground KGB command posts, and to evacuate the Politburo and the generals from Moscow in the event of nuclear war. Work on it has continued in secret ever since."

"I've heard the rumors," Charlie says, looking around them. "Everyone has. But I thought it was just dezinformatsiya."

Richard smiles. "You know what they say. The greatest trick the Devil ever pulled was convincing the world he didn't exist. That's the KGB all over."

In answer he nods toward Richard.

"So what happens now?" I ask Tolya, who has yet to utter a single word.

So we stand there, Richard dressed like a commuter bound for a day's work at a London investment bank, Charlie and I zipped into our black cold-weather gear like skiers at an Alpine resort, and Tolya looking like a Mafia enforcer.

"It's very simple," Richard says. "We wait for our train."

"So if the D-6 network is a secret Russian government asset, how do you and the Twelve have access to it?"

Richard frowns thoughtfully. "Eve, there are things I'm not at liberty to explain. Let's just say… It's complicated."

He's saved from further explanation by the arrival of the train. It's a single carriage, clearly many decades old, with an electric locomotive at each end. We climb aboard. The interior is functional but worn, with a single flickering light, threadbare upholstery and discolored windows part-covered by curtains. We sit down, the doors close with a faint hydraulic hiss, and the train draws away from the platform into the darkness.

Soon we pass through another station -- I glimpse a sign reading D6-VOLKHONKA through the grimy glass -- but the train doesn't stop until we reach D6-CENTRAL. The whole journey has taken less than ten minutes. Stepping from the train, regretfully in my case, we exit into an atrium very like the one at Efremova, except that this time there are half a dozen GUSP officers guarding the deserted station. In the place of Efremova's stairways, a succession of escalators rises within the steel-walled shaft. It takes several minutes to reach the top level, where we alight into a dusty, littered hallway with several exit corridors radiating from it.

Richard leads us to the furthermost of these, which is signposted NIKOLSKAYA. There's a light switch on the concrete wall though he ignores it, preferring to follow the pale beam of his torch. I can feel a cold breeze and the beating of my heart.

"Remember this journey," Richard tells Charlie and me, as Tolya looks on silently. "No one would believe you if you told them you'd ridden the deep rail. They'd think you were crazy, or a fantasist."

"This is where I say goodbye, good luck, and good hunting," Richard tells us. "Tolya, you know what to do."

The corridor goes in a dead straight line. There's smashed glass on the ground and puddles of dark water. At one point the torch beam catches a pair of shining eyes, and a cat bolts out of the shadows. Finally, we reach a dead end. There's an aluminum stepladder leaning against the wall, which Tolya stands up and climbs before pushing open a steel hatch over his head.

"You OK?" Charlie whispers, not unkindly.

Tolya gives us a couple of minutes to acquire our night vision. "OK," he says eventually, speaking for the first time. "More climbing."

"Yep. Thanks."

We make our way upward in near darkness. We're inside a tower, musty-smelling and ancient. A narrow stair ascends through three wooden floors, past high Gothic windows through which bright lights are visible, to a small, eight-faceted chamber. The windows are narrow and have not been cleaned in years, and several of the smaller panes are cracked or missing, admitting ice-cold air and the sound of singing and shouting.

Tolya nods, and effortlessly pulls himself up into the darkness. Charlie follows. I climb the ladder, jam my elbows through the gap, and with Tolya's help manage to haul myself onto a cold stone floor, where I collapse for several seconds.

I peer outside. Some sixty meters below us lies the glittering, illuminated expanse of Red Square, teeming with New Year revelers. On the far side of the square is the GUM department store, its towers and turrets strung with golden bulbs, and in front of the store, shimmering beneath a bank of spotlights, is an outdoor ice rink, around which skaters are whirling, weaving and occasionally colliding as pop music booms from loudspeakers. In any other circumstances this festive scene would be intoxicating; tonight, it's terrifying. It's like my first glimpse of the stage on which I am to be performing a leading role, despite knowing none of my lines.

The D-6 underground railway, I realize, has enabled us to bypass multiple layers of security checks and CCTV surveillance, and insert ourselves, unseen, into the interior of the Kremlin itself. I calculate that we must be in one of the historic towers on the eastern wall. On the floor, in a hardshell case, is the AX rifle, the Nightforce scope, and a suppressor. Beside it is a box of Lapua.338 Magnum rounds, a Leupold spotting scope in its case, two wireless headsets, a thermos flask, and a plastic sandwich box containing sandwiches, a chocolate bar and caffeine tablets.

"In your location, there are eight windows. With your back to the entrance, note the window at eleven o'clock. You will see that two of the lower panes have been removed. You will direct your telescopic sights and spotter scope through these."

"Echo ready."

As Charlie sets up the rifle, and I busy myself with the spotter scope, Tolya switches on one of the headsets, speaks briefly and passes a set to each of us. There's ten seconds of dead air, then a flat, disembodied voice requires us to identify ourselves as "Charlie" and "Echo." We do so and are told to prepare our equipment and report when ready. Tolya then wishes us good luck, and takes the narrow stairway downwards to the floor below to keep guard. The thermos contains hot sugared coffee, and I pour myself a cup.

"Charlie ready."

"Done."

"Opposite you is a red-brick museum with white turrets and roofs. From your position, draw an imaginary line to the ridge of the highest roof. You have approximately one meter clearance, minus the depth of the snow on the roof. Tell me when you have done this."

"Done."

"Negative."

"Echo, give me your range to the central pillar."

"Very well. The time now is nine minutes past six. At half past seven the car carrying the two targets will descend the one-way street running alongside the eastern side of the theater. You will be given warning of its approach. It will halt by the eastern pillar, and the targets will exit the car and walk behind the pillars to either the first or the central doorway. Your target is the Russian. Repeat, your target is the Russian. There will be bodyguards and others with the group, so correct identification is paramount. You will have, at most, fifteen seconds in which to identify and dispatch your target. One shot, one kill. Heard?"

"Charlie, confirm."

"Charlie seen."

"Yes, Echo seen."

"Done."

"Seven hundred and thirteen point five three."

"Done."

"Confirmed."

"Echo, how is visibility?"

"Continue along that line for four hundred meters, between the high buildings, and you will see ornamental gardens on your right. Cross the highway and your line cuts through the northeast corner of a square with a circular fountain at its center. The last hundred meters takes you to the front of a building with eight pillars in front of three double entrance doors. Do you see?"

"Excellent."

"Crosswind?"

It gets colder, and colder still. We finish the coffee, and Charlie takes a caffeine tab. "I'm glad we've got Stechkin."

"You know what that building is?" Charlie asks. "The one with the pillars?"

"In the sandwich box, then."

"Good. Keep this channel open. Remain at the firing point. Remain silent and vigilant. Be aware that you are potentially visible from below."

"Echo heard."

"Obviously."

"A theater?"

"The theater. That's the Bolshoi."

"Because he's shorter than Loy. They've given me the harder target."

"So pee," Charlie says.

"Fuck's sake, Eve. As if I'm interested."

My elbows and knees gradually lose all feeling on the floor. "I'm desperate to pee," I say, after a time.

"Charlie heard."

"Where?"

"Anywhere. On the floor?"

"It'll go through the boards. Onto Tolya."

"There's isn't room."

"So Oxana's got Loy?"

By the time I've finished, Charlie's eaten all the sandwiches and half the chocolate bar too. "What the hell?" I ask, zipping myself up.

"Why?" I ask.

"There is if you take the sandwiches out."

"OK, don't look."

"Thanks a lot."

"Self-control. In Russia we don't have this culture of instant gratification. Finish the chocolate, Eve."

"Preventative measure. When the moment comes, I don't want you wriggling about and telling me you need a shit."

"Fuck off, Charlie, you're just greedy. What about you needing a shit?"

"That's history, Charlie. Right now we've got a job to do."

"Pleasure." Charlie twists toward me, and grins nastily. "You're just pissed off because I did your girl."

Looking outside, I can see that this is not going to be easy. More and more people are arriving every minute, shouting, jostling and singing. In an hour Red Square will be packed solid. Every few minutes a snowball traces a slushy arc over the heads of the crowd, to be greeted with shrieks and laughter. From further away I can hear ragged cheering, the crackle of fireworks and the pounding bass of Dima Bilan's latest hit.

My voice is steady, but fear is coiling in my guts. I've given up thinking that there's going to be any intervention by Tikhomirov, or any stopping of this thing. The process has begun. All I want now is to do what we have to do and get out fast.

"So do you know how to get back to the building where we've been staying?"

"Tolya will take us."

"Yes."

"Tolya knows. Right now I need you to do your job and check for crosswind."

"Charlie, talk to me. What's the getaway plan?"

I inch closer to the right-hand of the two windows with missing panes, making sure that neither I nor the vapor of my breath is visible from below, and gaze along our line of fire. The Lapua round will fly at a declining angle over the roof of the museum, clear the banked snow there by less than half a meter, thread between two monumental nineteenth-century blocks, traverse two squares and ornamental gardens, and find its target on the pillared frontage of the Bolshoi. Through the Leupold scope I can see people lining up to make their way through the theater doors into the entrance hall, almost half a mile away. The optics are so fine, and the night air so cold and clear, that I can see the expressions on their faces. I can even read the posters announcing the evening's performance. Schelkunchik. The Nutcracker. I lower the scope and everything is miniature again and the Bolshoi a distant white matchbox.

"Do you know how we're supposed to get out of here?" I ask Charlie.

"Standing by."

"Seven hundred and fourteen point nine."

"Targets approaching. Two minutes from destination."

I sense Charlie settling into the weapon, stock to shoulder, cheek to cheekpiece, eye to scope. I can hear their slow, controlled breathing through the headset.

At quarter past seven our control comes back on air. "Targets en route, currently ten minutes away from destination. Stand by Echo, Charlie."

I try to calm my heart. Breathe in, hold for four counts. Breathe out, hold for four counts. Breathe in… It's not working. My heart's punching my ribs, my mouth is dry, and my neck aches from peering through the Leupold. I scan the target area. The walkway outside the theater has been cleared of people. The left and right entrance doors have been closed. A deputation of three men and a woman waits by the center door.

Charlie loads, the snick of the AX's bolt action barely audible, and settles themself, while I briefly aim the Leupold scope at snow-covered bushes five hundred meters away. There's not a tremor, not a flutter of a leaf. We have perfect, windless conditions.

"Can you confirm head-shot range on that middle door?" Charlie asks me.

"Car has stopped. Prepare to execute."

Behind the pillars, the group moves fast. Through the scope I catch glimpses of Stechkin, with his asymmetrical gunslinger gait, and the implausible blond swirl of Loy's hair. As they approach the delegation at the door both men halt. Stechkin's profile is in clear sight.

Stechkin is out first, and stands beside the car door for a second as Loy steps out after him. Then both are obscured by bodyguards as they approach the entrance and climb the side steps. "Wait until the door," I tell Charlie. "They'll stop to shake hands."

"Send it," I murmur, my voice weirdly calm, but Stechkin slips from view. From below us, partially muted by the headset, comes a sound like fireworks, and then there's a thump of feet on the stairs. I freeze, Charlie turns, and a burst of automatic fire smashes into their chest. Behind us, weapons leveled, are three men in FSB combat dress. From behind them steps a fourth and obviously female figure, in a black ski jacket and ski mask. Approaching Charlie, who is writhing and gasping in a spreading pool of blood, she pulls off her ski mask and draws her Makarov handgun. "This is for Kristina, bitch," she says, and fires a single round between Charlie's eyes. She watches them die, then looks at me bleakly. "Eve."

"They had to be. I knew as soon as I got your report. For which thanks, by the way. You were brave and brilliant, and I could not have asked more of you." He reaches out his hand, and mindful of the sad, bloodied figure of Charlie on the floor in front of us I shake it.

She's alive, I tell myself. She's alive.

"We're square," says Dasha, unzipping her jacket, holstering her gun, and giving me a tight, pale smile. "Thank you all, and goodbye."

"We thought we'd worked out where the second firing point was. I've got a team there right now, but there's no one there."

Tikhomirov inclines his head. "Goodbye, Miss Kvariani."

As she's leaving, Tikhomirov's phone sounds. He listens for a minute, mutters something inaudible, and shakes his head.

The three FSB men help me to my feet. I'm shaking so much I can hardly stand, and when we're joined in the overcrowded octagonal room by Vadim Tikhomirov, I just stare at him.

"Where's Vorontsova?" he asks me.

"Then we're square," he tells her.

"I don't know."

"How did you know they were the Twelve's targets?" I ask him.

"Dead?" Tikhomirov asks Dasha, indicating Charlie, and she nods.

"The good news is that Loy and Stechkin are safely inside the theater," he goes on.

"Dasha."

Taking my wrist, he leads me more or less forcibly into a dark, arched passageway, and from there through a massive, studded door into Red Square. It's jammed, and the dazzle of the lights, the blare of pop music and the acrid smell of fireworks envelop me in an instant. Tikhomirov pulls me through the crowd past a set of road barriers, to where a black SUV with FSB insignia is waiting. His assistant, Dima, is at the wheel.

"And now, while my men clear this place up, I should get you to a place of safety."

I attempt to pull my shocked and scattered thoughts together. "Two things, I guess. First, that your intervention here has proved them right, that I was an informer, and second…"

I follow him down the stairs, past the lifeless body of Tolya. When we reach the ground floor he opens a door for me, and then, frowning, closes it again.

His voice hardens. "Say it."

"Teatralnaya," Tikhomirov orders. "Go fast."

"Go on, Eve."

"Exactly. And there's only one place that's likely to be. Where the intended victims are. The Bolshoi Theatre."

I whisper it. "That the real attack is happening somewhere else."

"Let's suppose, just for the sake of argument, that there is no second firing point. That the whole idea of two sniper teams is, and always has been, a ruse. A diversion, sold to you in the knowledge that you might be an FSB plant. What then?"

"Second, that…"