"Focus." Nowwas the time for getting out, not speculating.

Madeleine began to walk, holding her phone up high in case ofsomething more unexpected than dust. Thearea between the rails was easy to walk on, with only stray lumps of clinker tolook out for, and she followed the gentle curve until the only sign of dustwere sprinkles which may have come from those who'd gone before her. Stopping to study a dusty print, shesuddenly found her coating of grime intolerable.

Shedding her backpack, Madeleine pulled loose the wooden pinshe used to hold her crinkle-curling brown hair in a knot at the nape of herneck, and ran her fingers through it over and over, showering an enormousamount of dust onto the rails. She waswearing a strappy sun dress, chosen because of Tyler, and not something she'dordinarily wear while painting. Shakingand patting it with her hands added to the cloud around her, and she moved afew metres further before trying to beat her backpack clean.

It was impossible to get it all off, but she did manage toreduce her coating to a light powder, and cracked another bottle of soft drinkto sip as she walked, fighting off the persistent itch in her throat. The clinker crunched beneath her feet, andoccasionally she heard sounds which made her pause, poised to run, tellingherself it was only rats, and far from reassured by that since she hatedrats.

Aliens or rats, whatever it was stayed away, and eventually apoint of light appeared ahead and the tunnel began to lighten. Soon Madeleine didn't need her phone to findher way, and she picked up her pace even as she noticed a fine layer of powdercovering the track and clinging to the walls. Circular Quay was not an underground station, and a thin coating of dusthad settled over it, including on the train – a double-decker Tangara type, big and blocky – which sat on the track atthe station platform. Fortunately it wasnot right up against the tunnel exit: first came a short section of track likea bridge, with a walkway along the side. Madeleine stepped up on this, and immediately looked out to what shouldbe a sweeping view to the Sydney Harbour Bridge across the ferry terminals.

The only trace of the Bridge was a dim grey line. Years ago a great storm of red dust hadpicked up in Australia's desert heart and swept across New South Wales all theway to Sydney, blanketing the city in a fiery haze. Madeleine had missed it, had woken only to afamily car which needed a good wash, but she'd seen pictures of the Bridgehidden almost as completely as this. When her mother had told her that a tower in Hyde Park had let out acloud of dust, she'd imagined a billow of smoke building to a cumulonimbus,something with edges. Not an entiredesert's worth of haze, to hide all landmarks and coat every surface white.

In the muted sunlight she noticed a faint purple tint to thecloud, and the whole thing sparkled, brighter motes catching the eye as theydrifted. An alien attack which came inshades of lavender. Beneath this pastelblanket lay a city hushed, unmoving. Usually there were buskers playing down in front of the ferry terminals,their music threading through the chunk and clatter of trains and the rush ofcars from the Cahill Expressway above. Today Madeleine could hear only a hum from the Tangarasitting at the platform, and maybe one or two cars creeping at a snail's pacealong the road overhead.

Slipping around the metal gate which divided the walkway fromthe platform, Madeleine headed for the escalators to ground level, glancing atthe train's lower row of windows as she moved. Through the film of dust she met the eyes of a half-dozen people staringup at her.

Their open horror made her flinch and for a moment she had aclear and exact picture of how they must see her. Not a skinny teen with big green eyes andhair on a life mission to frizz, but someone coated head to foot in unknowndoom. Dead girl walking.

What was the dust doing to her? It itched against her skin, tickled herthroat. Did her back and head achebecause of bruises, or was that the first symptom?

But Madeleine was almost glad not to be like those who staredup at her. She had escaped the wreck ofSt James, and in a way gained a second release due to the certainty of herlevel of exposure. The dust cloud wasnot a barrier to someone who had waded through the stuff, and she was notlocked in an air-conditioned bubble, hoping the train's guard had closed thedoors before any dust drifted inside. Would air-conditioning filter the dust out? How long would they stay there, unable to doanything but wait?

Head held high, Madeleine walked past two more carriages, andtook the escalator down to street level. She'd lost her ticket, and had a moment as she wriggled past the barrierwhere she thought she could remember being thrust sideways, falling, and thenshe was out, walking through a ghost town powdered white.

In the hour since a tower of black had arrived at St James,the usual crowds of Circular Quay – tourists, office-workers, shop staff, ferrypassengers – had vanished. Only theseagulls were out, shaking pale lavender wings and fighting over a spill ofabandoned potato chips. But, asMadeleine found her way below the overpass and headed east, she realised thatthere were people everywhere. In cars,the windows wound up tight. Peering outof hastily closed shop fronts and restaurants. Crowded in tight, anywhere there was a door which could be shut, wheregaps could be blocked with t-shirts or newspaper, where they could pretend thedrift of white-purple had been safely kept at bay. Like the train passengers, waiting out someunlikely Sydney snowstorm. Trying not tobreathe.

With visibility of no more than a few metres, it wasdisorienting walking through the cloud, but Madeleine was fairly certain she washeading in the right direction. A sirenmade her jump, and she turned sharply, only seeing the cloud and her footprintsin the settling layer of powder. Theblast didn't belong to any vehicle, but seemed to be coming from all aroundher. As she moved on, she began to makeout words, and realised it was some kind of emergency broadcast, though shecouldn't see the loudspeakers.

"...side...threat has been...panic...to seal...shutdown...do not go...hospital...damp cloth..."

The snatches of instruction came and went, followingMadeleine up to Macquarie Street, trailing her along the spiked metal fence ofthe Botanic Gardens, and fading completely as she neared the eastern border ofthe parkland known as The Domain and found the stairs leading off the promontorydown to Woolloomooloo. The dust cloudwas starting to thin and she could see a good portion of the seaside suburbbelow. Bracketed by two peninsulas – onepark and one naval base – the bay was narrow and entirely dominated by FingerWharf, with its long stretch of teal and white apartments, and row ofimpressive boats moored alongside. Thewater was as pale as the choking sky, a sluggish swell only occasionallybreaking the surface layer of dust apart. It made Madeleine wonder how far west Sydney's dams were.

A row of compact, expensive restaurants sat at the street endof the Wharf, their outdoor seating areas an icing-dusted display of half-eatenmeals and overturned chairs. Everyshutter was closed, every door sealed, and through the glass she could see morecollections of the trapped, crowded together, sitting on the floor, huddled indespairing clumps. Staring back at her.

Even when the cloudsettled, the dust would still be everywhere. How would anyone get home without kicking it up? How could they get rid of it all?

There was at least no difficulty getting into Tyler'sapartment. The electronic key to theresidents' section of the central walkway gave her no trouble, and then she wasunlocking his door, dropping her backpack, suddenly in a hurry to turn on theshower, to stand fully clothed in a blast of steaming water and watch herviolet dress return to its original white and blue. A trembling weakness followed, becauseshedding that powder coat left her like the others: trapped and fearful. All she had now was the wait for the dying tostart.