A breeze stirred the air outside, and a few of the creatures came to a stumbling halt. One thickset fellow, his filthy face partly shrouded by a scraggly tangle of hair, looked up and towards us, then staggered in our direction a few paces before coming to a halt. His eyes were black, with hardly any white showing, every trace of humanity gone.

‘Let me see that,’ said Nadia, snatching the binoculars back. She stared silently ahead for several seconds. One or two other bee-brains had also come to a halt, swaying their heads as if they were sniffing at the air.

The breeze died down. All of a sudden they appeared to lose interest in us and began to shuffle off after the rest.

‘Well,’ said Nadia, lowering the glasses from her face, ‘if that ain’t a first. Did you see that, Oskar?’

‘Yeah,’ he said. ‘It was almost like they were looking our way.’

‘Could be coincidence,’ she said. ‘Them being completely brain dead and all.’

‘They don’t look that dangerous,’ I said.

‘There are three of us,’ said Nadia, ‘and a couple billion of them scattered all over this alternate. That means they win any fight by a knockout, even before they get in the ring.’

The last of the bee-brains moved out of sight. I felt my skin prickle at the thought of the people those strange, shambling figures had once been. I felt the irrational urge to breathe shallowly, as if it might protect me from a whole cornucopia of imagined airborne infections.

‘Here we go,’ said Nadia, putting the SUV into first gear. We trundled slowly forwards and across the now deserted intersection.

Nadia pulled a walkie-talkie out of the glove compartment and handed it back to Oskar. ‘Check in with Casey and the rest,’ she said. ‘See about getting an update from base camp, see if the recon drones have spotted anything unusual. I don’t like the way those bee-brains were acting.’

I studied the map while Nadia navigated us past a number of abandoned vehicles blocking the road, before we accelerated towards an on-ramp that linked to an elevated motorway. Soon, we rose above the dark waters of a river that cut through the heart of the city. She slowed as we approached a row of vehicles that looked to have been arranged in a deliberate barricade across the motorway, carefully edging the SUV between an overturned bus and the edge of the bridge. I caught sight again of the river below, which, according to the map spread across my knees, was called the Pinheiros.

‘Hey,’ said Oskar from behind me, jiggling the walkie-talkie. ‘I’ve been trying to get through, but I can’t.’

‘Not at all?’ asked Nadia.

‘Nope.’ He turned the walkie-talkie upside down and poked at its battery compartment. ‘Don’t think there’s anything wrong with it. Maybe there’s some kind of interference.’