Eddie looked at Nina. ‘Should we?’ She didn’t need to speak for him to know her answer. ‘Oh, sometimes I fucking hate being the good guy! Get the door open!’ He ran back down the passage as Nina rushed to the exit.

The cloud’s hazy edge rolled ever closer to Eleanor. She tried to drag herself away, but lacked the strength to do more than slither clear of the wheelchair.

Eddie reached her, holding his breath – and hoping the poison was not absorbed through the skin. He scooped her up as yellow tendrils reached her feet. She was shockingly light, useless legs withered to sinew and bone. He raced back towards the corridor’s end.

Nina reached the door. It had a handprint scanner like the elevator. ‘Eddie, it’s locked!’

‘Aren’t you glad you saved me now?’ Eleanor rasped. ‘Hurry up and get me to the door!’

‘I’m not your bloody butler,’ he replied. A glance back. The grimy yellow cloud swallowed the wheelchair. ‘You just get your bony hand ready!’

‘Come on, come on!’ Nina shouted. ‘Quick!’

‘Oh great, now I’ve got it in stereo!’ Eddie complained as he arrived. There was no time for gentleness or even courtesy; he flipped Eleanor’s legs downwards, one arm around her lower chest to support her as he brought her to the scanner. She gasped, but pressed her hand against it.

An agonising pause – then a hefty bolt retracted. Nina hauled the thick barrier open. A bare concrete shaft lay beyond, stairs sloping upwards at a steep angle. The top was an alarming distance away. ‘We’ve got a hell of a climb!’ she said.

Eddie threw Eleanor over his shoulder in a fireman’s lift, ignoring her angry protestations as he started up the stairs. ‘And I thought living eight floors up was bad enough when the lifts are out.’

‘It’s good for your butt, though,’ said Nina behind him, regarding said body part.

The old woman made a disgusted sound. ‘Please, spare me.’

‘We already did,’ Eddie reminded her. ‘You want me to leave you here?’

‘You’d regret it. You’ll need my handprint to open the top door as well.’

‘Does the rest of you need to be attached to it?’ Eleanor fell silent.

The forced pace of the ascent was exhausting. Nina looked back. Amber warning lights flashed at the bottom of the shaft; the gas had reached the emergency exit. But the climb’s end was in sight. Leg muscles burning, they finally arrived at a landing. Another door awaited them, this time a large steel one set in reinforced concrete. Eleanor’s bunker had been built to withstand even the most determined intruders.

‘Okay,’ Eddie wheezed. ‘Get this . . . bloody thing open.’ He slid Eleanor down.

Even though she had not been running, the jolting journey had left her breathless. ‘Just a moment,’ she said between inhalations. ‘I need some air.’