There was an order to it. The bar inside had to be raised again and the metal latch caught before it would close. They got it wrong the first time, and the second time too until the boy held the door to reach up and set the mechanism up right.

The corridor was on fire. Mary heaved as hard as she could, and felt the bolts catch. She staggered back, and caught her first sight of the façade of a glazed red-brick tube station entrance embedded in a lone stack of rock, the endless ocean behind it.

She fell on her backside. The sea rushed in quickly, so that it didn’t matter how nimble she was at standing again. The moving water dragged at her, and she had to hold on to the black boulder, studded with sharp white shells, to stop herself from being swept away.

She looked up again at the lintel, where the weathered letters were picked out in cream: Down Street. Above that was the hint of an arch, where it seemed to merge into the natural rock. The door was still there, surrounded by bricks. Painted grey, with a blue sign fixed to it.

Even as she read the writing on it, it faded and grew more indistinct. Now she couldn’t tell if there had been a door or not, whether the bricks were simply jointing in the rock, whether the letters over the door were just a trick of the light.

Then it was gone. No more than a slab of wet rock, imprinted with a vague impression of a door. If the light changed, or she looked away, even that would be lost.

A string of orange boilersuited figures stretched from the lone stack in the sea, along the half-covered blocks, to the rocky foreshore.

The boy’s turban was deep blue, dirty and scorched in places, wet with spray. He had a sort-of beard, one she’d mistaken for soot stains. He was staring back at the place the door had been, and he, like her, couldn’t quite believe anything anymore.

‘Are we dead?’ he asked her, even as the water washed up his back and made him shiver. ‘We should be dead, right?’

A wave caught her, making her body rise and she lost her grip. As she scrabbled at the rough rock for another handhold, she grazed her palm on a jagged edge, and her skin split. Blood bloomed in red beads along the cut, and when she was able to get her feet down again and brace herself against the pull of the sea, she inspected the damage.

It wasn’t deep, but the salt water stung. She turned her hand palm out as evidence.

‘You might be dead. I’m not.’