"Tom!" Andrew's face broke into a relieved smile, and he pedalled toward his friend who was weaving his way through the randomly parked cars on the dual-carriageway. Alan the dog was held on a short piece of rope and Tom smiled wanly back at them. His face was masked by grief, and dark shadows hung under his eyes.

Andrew parked his bike and hugged him fiercely, joined a moment later by Eddie.

"Hey guys," he said. "It's good to see you."

As he disengaged from his two friends Merryn took her turn to hug him, and it was then that Tom broke down, deep sobs shaking through them both as he clung to her, Alan whining at his feet.

"Come on," said Merryn after a few minutes, "let's find a spot off the road and get some food into you."

Tom nodded and a few moments later with a mug of hot soup in his hand he filled them in on the details of Marcia's final moments.

"She was one tough old lady. I climbed to the top of the Tor to the north of the camp before I made my way down onto the road. A swarm approached from the west, and just seemed to roll over the camp. It didn't even change pace really. A few walkers milled around for a bit while Marcia was still alive and firing, but the rest just kept going. The fences were fine against one, or even a few deadheads, but the swarm hit the fence, then the zombies behind walked up over the other ones and kept rising until they got over the fence.

"Marcia was in the same tower she was when we found her. She was firing away for about five minutes and you could hear her shouting at them at the time. For such a nice lady she had a pretty impressive repertoire of swear words. But, they kept rising up, building up a pile of deadheads until they reached her. There was a single shot from a pistol, then a massive boom a couple of seconds later as her hand grenades went off. I don't know how many she took out, but it wasn't enough. I'm ahead of them, but not by a long way. Didn't have time to sleep."

Merryn rested a gentle hand on his shoulder. "You did everything you could, Tom. Marcia would have been happy you got out and took care of Alan."

"I couldn't save her," he said, tears once more welling up in his eyes.

"That's not a lady who needed saving," said Eddie. "She went out on her terms. Marcia would have been far more interested in saving you and Alan, than needing to be saved herself."

Tom nodded and wiped his sleeve across his eyes. "Thanks, Ed. So what now?"

Merryn straightened up from where she'd been making a fuss of the dog. "You and Andrew are going to get back to base. Take two of the bikes. That one's got a pannier on it that Alan can go in. Eddie and I will take the other one and scout ahead. We have a radio, so we'll report back to base and let them know what's going on. You need to report straight to Captain Summers and brief him on what you know. He's making plans for defence, and he's assuming the swarm is going to hit them. If it doesn't then that's fine, but we need to know where it's going."

"May I suggest an alternative?" said Andrew.

Merryn cast him an amused look. "I'm assuming this is an 'I've got a better idea' moment?"

"Well, yes." Andrew grinned at her. "Back at the last junction there was a sign for bike rental. I can't imagine they've all been plundered, so why don't Tom and I go back there with one bike? I can cycle back to base, Tom can have the electric one with Alan as they're knackered. That leaves both you and Eddie with an electric one so you'll be more mobile."

"Yup, better idea, but if you get stuck, come back to us here," said Merryn. "Okay, get going you two, Eddie and I will move out west and see what we can see."

Merryn watched as Tom and Andrew moved east, got on her bike, and nodded at Eddie who was already mounted and ready to go.

The two of them took their time, moving with care around parked and crashed vehicles and making steady progress along the road. At the top of a hill on the edge of the moor, Eddie, who was a little ahead, slowed, then stopped, reaching for her binoculars. Merryn joined her and did the same.

"Tree down," noted Eddie. "Halfway down the hill on the left. It wasn't like that last time we were here was it? And there's our swarm coming over the top of the next rise now."

Merryn swung her binoculars up to the top of the next hill and swore under her breath. The swarm was spread out over some distance, some groups seeming to wander within the main group as if disconnected or confused.

"Is it me or do they behave like lots of smaller groups who happen to be going in the same direction rather than one large group?"

"It does seem a bit Brownian motion doesn't it," muttered Eddie.

"Do what?"

"Sorry, random, not structured."

"Quietly, you're a hell of a lot cleverer than you make out sometimes," said Merryn.

"Andrew says that too, he keeps telling me to speak up more."

"He's absolutely right," Merryn cast her a sideways look. "You two look like you're getting on well."

Eddie blushed. "He's kinda cool."

"He is, if a little random sometimes. You're a nice couple and I'm glad you've found each other. But at this present moment in time we have something else to worry about. How many do you reckon?"

"More than the swarm that went past the base, quite a lot more. Ten thousand? They are working their way in this direction. The road is an easier path than the woods or the moor, and the road embankments are funnelling them in."

"So once they reach the more open areas of flat farmland maybe they'll spread out a bit."

"Maybe."

Merryn lifted her binoculars back up to her eyes and scanned the road ahead. "Eddie, the tree down there you pointed out, is that where the car full of odd grey goop was?"

Eddie lifted her own binoculars and stared ahead for a moment. "Yes, I think so. That people carrier had hit the tree. It's a heavy car so it wouldn't surprise me if it damaged it enough to weaken it. Should we go and have a look?"

"Yes, but don't get too close. Let's stop a good distance away and use our binoculars again."

"Okay."

The two of them maneuvered their way down the hill through the non-moving traffic until they could see the tree more clearly. It had folded down over the car that had struck it, several branches impacting the car. The front window was cracked but still in place, but one branch had impaled the car straight through the sunroof.

Grey sludge had worked its way up out of the car, pooling across the metal and glass and down onto the tarmac below, spreading across both lanes of the carriageway. It seemed to ripple and change, moving like metal filings over a strong magnet with patterns coruscating across its surface.

On the far side of the car, a lone walker was held, statuelike in the goo. Both feet were welded to the ground and its arms were outstretched, mouth agape and unmoving as the goo moved around its body.

"What the hell?" muttered Merryn.

"It's like a venus fly trap," said Eddie. "But what's it doing to the walker?"

"I don't know, I dread to think. But I don't want to get any closer to it." Merryn shuddered. "Take some pictures, catalogue it. That's all we can do for now."

She looked west down the road at the shambling horde that walked towards them. They were quiet at the moment, and all they could hear was thousands of feet marching in broken rhythm toward them.

"Check our route out please Ed, I'm going to keep an eye on this mob for a moment."

Eddie nodded and cycled back to the top of the hill. Merryn withdrew halfway up and stopped again. The front edge of the shambling mob was edging closer to the fallen tree and she lifted her binoculars to her eyes.

A walker stopped, then another, then several more, each hitting the patch of grey shifting goo on the ground. But the walkers behind them didn't stop. More and more zombies piled forward, and soon a wall of deadheads formed, spreading north and south as other walkers were forced to try and go around them. A silver grey stain spread from the front of the pile at ground level, rising upwards in tendrils of moving liquid, moving like mercury through the ranks of the undead, quicksilver traces freezing the walkers where they were.

"What the hell is going on?" said Eddie, startling her. She stood next to her, eyes wide, and skin pale.

"It looks as if they're being infected. That silver grey goo is spreading fast, covering them. It's..."

Her words were drowned out as thousands of walkers gave voice at the same moment, a collective moan that drove birds from the trees, lifting the hairs on the backs of their necks and setting their hearts pounding in fear. The pile of deadheads started tumbling apart as the walkers moved again, and they spread out, moving apart from their fellows until a solid mass of the dead stood before them, standing shoulder to shoulder, packed like sardines between the long disused vehicles.

"We need to move back," said Merryn, her voice shaking, and the two of them moved to the top of the hill.

They stopped and looked back at the crowd below. "It's like someone's parked them there," muttered Eddie. Merryn was rummaging through her backpack and set up her radio on the bonnet of an old Land Rover.

"Control, Scout 2 reporting in."

"Control, Mal here. Everything okay Merryn? You're not due to report in for a few hours yet."

Merryn relayed the events, and once she'd done, the radio crackled and Captain Summers came online.

"Scout 2, this is Summers. What are they doing?"

"They're standing. There's some movement at the back of the crowd though. I'm assuming that there's still some walkers joining and being... assimilated. That's the word I've been looking for. They're linking together somehow. They're standing facing the same direction: towards us. It's terrifying sir. Before now they've bumbled around, even if they were heading in the same direction, it's been somewhat random. Eddie mentioned Brownian motion whatever the hell that is.

"What would you like us to do sir?"

"Monitor for now please Scout 2." There was an audible sigh over the radio, and the Captain muttered something to Mal. "This goes no further than the four of us please Merryn. The scientists on the Isle have come up with a working theory about the goop in the car, and it ties in with what you've described there.

"They suspect the car of goop was one family, a large one given the size of the vehicle. We think they had a mutated virus, yet another mutation alongside the one that you and Hope have, and the other one that produced Stan. But this one appears to have somehow rendered them down into a connected organism, one that wants to spread, and it spreads through the walkers. So, a single walker would join it, but a group or swarm would become 'joined' at once. HQ is theorising this may lead to a super-swarm, like you have there, but one that acts in concert, a bit like a flock of birds turning as one, or a hive of bees that work together with one another to carry out an action or task."

"Thank you Captain. We'll keep ahead of them, and I'll try and get some estimation of their potential speed of movement and lateral size of the swarm."

"Noted Scout 2, thank you. I have one more ask. I'd like you to try and take out one of the new walkers. Use a silent weapon, get close, but not too close. We need to know we can still take them down."

"I'll do it," said Eddie without hesitation. "You stay here and watch what happens."

"Did you hear that Control?"

"Affirmative Scout 2, we'll stay on speaker, keep your microphone open for a moment please."

Merryn watched as Eddie threaded her way through the cars, crossbow already primed for a shot.

At a distance of about twenty metres Eddie rested her crossbow on the roof of a low sports car, took aim, and fired.

A walker on the front row hit the ground with a thud and lay unmoving. The response from the swarm though was instant and a low moan spread from the walkers close to the downed zombie, rippling back across the hillside and building in volume.

Eddie ran.