Jace looked sharply at me and then back at Tom, who peeled off a volley of shots with his automatic rifle, the bullets tearing into the Grey's flesh and practically ripping it apart mid-flight.

'Tom?' As in your husband Tom? Evie, are you fucking kidding?'

Before I could say another word – another foolish word – Jace was pushing on the glass doors.

'Jace, what the Hell are you doing?' I cried out, my heart pumping hard in my chest, as I tried to reach out and grab him before he could go out there.

He couldn't do this. He couldn't. He thought it was Tom. He thought he was saving my husband, when all he was doing was walking right into some kind of trap. It had to be a trap. There was no other explanation. Greys didn't turn on their own. They just didn't.

I saw flashes of the alien symbol crudely carved into Rico's flesh.

The way he'd locked his gaze with mine.

He did a good job, yes?

No. I refused to believe Rico. I refused to believe any of this. Nothing here was what it seemed.

'We've got to do something!' Jace insisted. 'We've got to help them!'

He was through the doors before I could do anything else, moving quickly to the edge of the terrace. The explosion of bullets from his SA80 tore violently through the front line of Greys, spraying blood and ruptured alien flesh into the air. Spotting a new assailant, the swarm opened their mouths, their awful high-pitched insectoid scream cutting through the air.

I could never decide which of the two terrified me more: the sound of the alien throat-clicking in the silent shadows of the tube tunnels or that scream which had the power to razorblade my stomach into pieces. Listening to it here, in the beautiful gardens of Lancaster House, with the sun shining like it was a normal summers day in the city, I don't think I had ever hated it more. That sound didn't belong here, any more than they did.

Lena and Tom began to run towards Jace, and I stood frozen in horror as the Greys continued to surge out of the trees and across the gardens.

'Oh my god,' I murmured.

There were so many of them. Countless numbers of Greys streaming towards us in a great monstrous tide. More than I'd seen since the days of the Final Wave.

Moving alongside Jace, I began firing off shots at the advancing army, cutting down those that were closest. By now, Lena and Tom were at the steps. The Norwegian's face was a mask of sweat and deathly pale skin as she struggled to climb up to the terrace, limping badly. There were deep lacerations in her shoulder from which she was bleeding profusely, and I could see the blood now dropping down the back of her hand, droplets falling from her fingertips onto the bleached stone. By her side, still holding her up, Tom struggled to keep hold of Lena and as they reached the top of the steps, Jace moved to grab her as I continued to shoot into the oncoming Greys.

'Evie, come on,' Jace called out over the sound of the rifle-fire. I glanced over to see him moving backwards with Lena.

Tom was now facing in the direction of the creatures, not more than a few metres away from my side, jerking his rifle towards a Grey as it reached the bottom of the steps and hurled itself at him. Stumbling backwards, Tom lost his footing, his eyes widening as the alien bore down on him.

Without thinking, I turned sharply, releasing a cascade of shots that ripped through the Grey's abdomen, sending it twisting and screeching across the terrace, a spray of blood arcing into the air.

Tom struggled to his feet, his chest heaving in and out, and our eyes met, just as they had that day across the ruins of the fountain in Piccadilly, just as they had across the table in the Maria Luisa, just as they had so many times before, and I was drowning.

Drowning, drowning, desperately drowning.

'Evie, for fuck's sake, get out of there!'

Tearing my gaze from Tom's, I ran towards Jace and Lena, not even bothering to see if Tom was following, but I knew he was. Even as we ran, fleeing the Grey hoard, I knew he was there, a ghost haunting my every step.

We raced into the Grand Hall, just as Taj, Len, Gav and Abby were streaming through the opposite archway, their panicked expressions looking weightier than the full backpacks they now carried.

'Go, go!' urged Jace, not stopping when he saw them.

'Who the Hell is that?' Taj demanded, as he threw one of the spare backpacks in my direction and gestured towards Tom who had rounded the corner just a second after me.

'Evie's husband,' snapped Jace. 'Don't ask, just fucking go! We have half of the Black Zone army right behind us!'

Their shock was palpable, a split second of uncertainty reeling through them all.

Now, Evie, tell them now. Not Tom. He's not Tom. He's not your husband.

Lancaster House screamed.

A terrible, shrieking noise punctuated by a what seemed like a thousand throat-clicks echoed through the hallways.

They were coming and it was enough to get everyone moving, all doubt and surprise cast aside in favour of surviving this new nightmare.

Jace and Lena were already out the main door and the others followed, charging out the entrance into the Stable Yard. I followed them out of the foyer, almost toppling off-balance as I struggled to haul the bag onto my shoulder. From the side, I felt a hand against my arm, holding me steady and I recoiled from it, spinning and hitting the doorway hard.

'Don't you touch me!' I hissed into the face of the Grey, glad that the others were far enough away not to hear. 'Don't you ever fucking touch me!'

The creature backed up a step, its alarmed gaze darting instantly to the way I was holding my rifle, my finger poised over the trigger.

Why did it have to look so like him? Why did I have to see the hurt and fear in Tom's eyes like I was the one who'd done something wrong?

Without another word, I fled, crossing the portico quickly into the yard, my feet kicking up dust as I ran, trying to push the image of Tom's face falling at the sound of the venom in my tone.

His pain was fake. He was fake. Not real.

In the courtyard, Jace was desperately trying to help Lena - who was limping now, perhaps a bad twist from the fall in the garden - while also furiously gesturing the rest of the group to keep going.

This was Plan B, after all. We wouldn't wait. We wouldn't look back. We would just run. Hide.

Survive.

No matter what it took.

I ran out and hooked an arm around her other side, prompting Jace to let fly with a barrage of curses.

'Fuck's sake, Evie,' Jace said, his face damp and reddening. 'I've got this. Get out of here, will you?'

'And leave Lena in your care? She'd be better off asking a Grey to help her to the nearest bus stop. I'm not leaving you.'

'This isn't the way we do things and you know it.' He was breathing hard as he tried to force out the words. 'You need to go with the others, now!'

Gunfire cracked through the air behind us, and I fell away from Lena's side, whirling around to see Tom tearing into the first few Greys that had come crawling out of the entrance to Lancaster House. Blood and flesh sprayed across the honey-coloured brick. Raising my rifle, I hit the trigger, managing to catch a Grey that had leapt clear of the first onslaught, taking out its legs. It crumpled to the ground, its black eyes still bleeding hatred and hunger as it tried to pull itself along. Before I could shoot again, a hail of bullets took out half of its skull, and what was left of the alien dropped lifeless to the dust and grit of the courtyard.

Confused and bewildered but with a fear-induced rush of adrenalin coursing through me, I refused to acknowledge what Tom had done and instead, turned and fled after Jace and Lena, who by now, had made it out to Cleveland Row.

To my dismay and surprise, I realised they weren't heading in the same direction as Taj and the others, instead they were cutting across the square towards Little James Street. Racing towards them, I reached their side just as they passed by the imposing building of the Sudanese Embassy.

'Where the Hell are you going? Green Park Tube is the other way,' I said, glancing back to see Tom not far behind, and over his shoulder, the heart-juddering sight of the Greys now swarming into the courtyard of Lancaster House.

'This... way,' Lena said, through gritted teeth. Her short blonde hair was plastered to her head, her skin starting to take on an alarming grey hue. 'Head for... Spencer House.'

'What? Are you crazy? We need to get into the tube tunnels!'

They were coming. I could hear their terrifying screeches growing louder as more of them took on the hunt.

'We would... never make it,' Lena said, her Nordic tones sounding more clipped as she struggled for breath. 'Spencer House... cellar. Get to the cellar.'

This was wrong. Everything about this felt wrong. We should have been going with the others, we should have been finding refuge in the tunnels, not running the other way with the intention of hiding out in the cellar of a building we knew nothing about. Especially when at least one of our new companions was most definitely not human.

With no way to go back and seemingly no choice but to push forward, I kept running, despite the doubt that gnawed furiously at my insides and despite the fact I knew, if I were to turn my head, I would see Tom running alongside me, like he had never died. Like I had never watched him get consumed by a Grey in a darkened alley. Like I had never spent two years mourning him and feeling like I was being consumed myself, not by an alien, but by madness and grief and guilt.

At the end of the road, Lena urged Jace to take a left, pushing him through a narrow gap where a once-white transit van blocked most of the entrance. The sliding side door was wide open, the inside of the van stripped clean of whatever it had contained. Dark bloodied handprints, now a rusty brown with time and sun, smeared the edge of the door. Hanging off the wall of the entrance, a battered, ash-covered street sign read 'Catherine Wheel Yard.'

By the time we reached the terrace of the once-splendorous building of Spencer House, I could see that half of the garden beyond had been obliterated, now nothing but a great cavernous sink-hole that looked as if H.G Wells' Invaders had torn it apart as they'd sought to escape from their earthly hibernation. The bombs had been merciless here, caring little for the eighteenth-century mansion, as they'd churned up carefully sculptured gardens and taken out the far wing of the house.

Lena guided Jace onwards, threading through a twisted maze of broken terrace furniture, heading towards an open doorway, half-barricaded by a stack of plush velvet armchairs. Her limp had noticeably worsened and she was clutching onto him, as if she would drop to the floor if he were to let her go. Looking back, I could see no sign of any Greys in the courtyard, apart from the one travelling so close I could have reached out and touched him, but I wasn't about to assume it would remain this way. The Greys knew we were here somewhere, and it wouldn't be long before their death squadrons were pulling this whole area apart trying to find us.

We couldn't go back, but I didn't want to go into this supposedly hallowed cellar that Lena seemed to believe was the answer to our prayers. Cellars always felt like dead ends to me. They were dead ends. Places where people went to die. We'd come across a couple in our time since the Final Wave and they were rarely places where we could walk out with backpacks full of scavenged loot. You walked out with images that never left you. Nightmares that plagued you. The last thing I wanted to do was end up in a cellar myself, waiting to be found, waiting to die, and I definitely didn't want to go down into the cellar with him.

With Jace already through the barrier, I took a deep breath and reluctantly swung my SA80 onto my back, climbing in after them and jumping down on the other side.

The room leading off the terrace looked like it had once been the library, no doubt a cosy, sumptuous room, encased by walls of leather-bound volumes. The mirror above the marbled fireplace was shattered, a spiderweb of cracks snaking out from one corner and reaching all four sides. A large oval oak table was upturned onto its side, the drawers open and ransacked, piles of books scattered across the carpet, pages torn and covered in muddy footprints. Gone was any sense of tranquillity here, now replaced only by an atmosphere of terror and chaos.

Passing through the library, we found ourselves in a smaller reception room, with a beautifully gilded apse above a double door archway. One of the gold and red brocade heavy-weight drapes lay on the floor, almost fully concealing the body underneath if it wasn't for the booted foot protruding from underneath, and the stench of rot that lingered still.

Out through the doorway on the other side was the entrance hall. A wide staircase led upwards and a large marbled centaur pointed to the skies, as if directing us to the next floor of the house, and not through the small door at the rear of the stairwell where Lena now led Jace.

'Quick,' she urged him. 'Through the cloakroom to the back... a door, another door...'

This was it. I could feel it. This was the time.

I waited until they had gone through.

I waited until I knew the Grey was right behind me, his close proximity too difficult to ignore any longer and I turned, whirling around and aimed my handgun directly at him. Clearly startled, the creature held his hands up instantly, his eyes wide, and I stepped forward, disarming him of his rifle which he relinquished surprisingly easily. Spurred on by his submissive gesture, I pressed the barrel against his forehead.

God, it hurt. It hurt to see his face. The way his hair curled at the temples when he didn't bother to style it. The infinite blueness of his eyes. To see him here in front of me, when I knew it wasn't really him, but an imposter. A ghost. A monster.

'Give me one good reason why I shouldn't blow your skull to pieces right now,' I hissed, deliberately keeping my voice to a whisper. 'One good reason, you murderous piece of shit.'

I pressed harder, enjoying the way he winced as I increased the pressure, enjoying the way his chest heaved in and out with fear. My finger brushed lightly over the trigger.

His fingers were brushing over my wedding band, and he was rubbing his thumb over the delicate gold ring, pressing his mouth against my neck.

Stop, Evie.

'I should kill you,' I said.

He slicked his tongue across his dry lips, his terrified eyes not leaving mine.

'You won't,' he said.

Oh God, he even sounded like Tom and I hated it and loved it equally because I never thought I would ever hear his voice again. I wanted to close my eyes and get him to say my name, to tell me he loved me, to say anything as long as I could just listen.

I had to be strong now. Had to.

'Shut up. Shut the Hell up.'

The creature sucked in a breath, but blinked slowly, a flicker of obsidian crackling across its irises.

Click-click-click.

My stomach flipped. It wasn't him. The sound wasn't coming from him.

Slowly, he moved his right hand, pressing his index finger to his lips, the direction of his gaze shifting upwards.

There was a Grey here. Inside the house. Above us.

Click-click-click.

Tom screwed his eyes shut as if he couldn't bear the noise, and swallowed hard, a pained expression on his face.

When he opened them again, it was like looking into someone else's eyes. They held a steely coldness that was so unlike Tom, that I recoiled from it, stepping back, my foot crunching on something on the floor behind me.

Up above, the clicking stopped abruptly.

Waving the gun, I motioned for him to move towards the staircase and he did, slowly, reluctantly, and I moved behind him, using him as a shield against whatever lurked up the stairs. Tom hesitated at the bottom step, turning back to look at me, pleading, but I held firm, urging him onwards. Whatever was coming, it was going to hit him first and who knew, maybe I wouldn't have to put a bullet in that face after all. Maybe one of his own kind would do the honour of killing him for me.

We eased up the stairs, cautiously treading each step like we were heading down into Hell, instead of following the centaur's directions.

A metallic creak echoed down, like metal on metal, and we both followed the sound, looking up where a beam of sunshine was streaming through the tall Venetian window, illuminating the high ceiling above in a haze of warm light.

The lone Grey screeched, launching itself from the caged metal chandelier where it had been hanging, in an almost-weightless leap, its long fingers splayed as it hurtled towards us.

A split-second of fear froze me to the spot, a stupid momentary panic disabling me as Tom jumped towards me, grabbing my waist with one hand, his other reaching for my own that held the gun.

I was dead now. My last seconds destined to be drained out in the stairwell of Spencer House, killed not just by the Grey now falling towards me, but also by the one that wore the face of my husband, who was now holding me from behind, preventing my escape.

I felt him there, pressed against my back, the warmth of his body a cruel reminder of what I had lost, a cruel reminder of how I'd let my love for that face stop me from putting a bullet in it when I'd had the chance. Now, it was too late.

Manoeuvring me slightly to one side, Tom gripped my waist, raising my other arm, his hand covering my own. His finger pressed down over mine on the trigger.

The shots cut through the oily-grey chest of the attacking alien, ripping a hole in its flesh before it could reach us, but I shrank back regardless, pushing myself against Tom, who still held me tight. The creature tumbled onto the landing halfway up the staircase, still screeching in agony, still alive, twisting and writhing in agony. Tom took aim again, steadying my hand and waited...

One, two beats of his heart against my back.

The creature took its chance - one final stand - and crouched, preparing to leap. With frighteningly skilful precision, Tom pressed down on my finger once more, and the bullet hit the Grey in the head before it even had time to raise its broken body from off the floor. It collapsed, lifeless, a gaping hole where one of its terrifying black eyes had been.

It was seconds before I began to breathe again. Seconds before I realised I was still locked into Tom's embrace, his hand still covering mine, but my arm now hanging by my side, the gun pointing at the floor. Seconds before I realised could feel his breath against my neck, his heart beating against my back, my own racing hard in my chest.

Repulsed by his embrace, his closeness, his body against mine, I pushed away from him violently, thinking about nothing but getting away from him. Shoving him hard, he fell against the wall but the force of my desperate attempt to escape sent me tumbling backwards, my foot slipping on the edge of the step. Crying out, I fell back, unable to stop myself and I hit the marbled stairs hard, tumbling, rolling, falling, feeling every unforgiving edge of the stone steps until I reached the bottom of the stairwell.

My head connected with the base of the great centaur and the pain exploded across my skull, a deep, resounding thunderclap of pain that tore through my head like a cluster bomb ripping apart the land, churning it up into a sinkhole where only monsters lurked.

The darkness rushed in quickly, but not fast enough to stop me from seeing Tom's murderer leaning over me, his head tilted oddly to one side, his eyes now completely black and that strange, chilling clicking noise emanating from his throat.