The spray can crackles its path across the discoloured soot-brick wall, spitting out a throwaway rag of stained colour, like a hand trailing an exotic scarf between dark fingers. An arc of searing hues bleeds out onto the bricks, dripping down to pool around my battered Converse trainers. There are purples like orchids, like a nurse's uniform, pungent reds and these bright, bright oranges, like the sun has liquefied. The bricks have vanished, replaced by setting suns, salmon cakes and honey dripping around rosy clouds.

Just then, the can emits a flat, hollow phut, like the sound of a gunshot equipped with a silencer, and the spray judders to a halt. I pause, and rattle it. Sweet ophelia-red leaks onto my hand, discolouring my fingers.

"Is it out?" Anya asks me. Her accent is heavy, like a shroud draped across her throat.

I shake my head, lift the can again and press the nozzle down. A fine spray seeps out, tingeing the air with rainbows. "Nah. Still some juice left," I say. "You remember the signal, right?"

She nods. "Beatles," she says, in a tone that's half-conformation and half-questioning, and glances towards me for reassurance.

"That's right." I add the finishing touch to my artwork – a stained-glass window, its rigid frames enclosing a spiralling explosion of symbols and rainbows and clashes and flaming golden letters – and pick up the black can. "You know how it goes, right?"

Her smile shows a piano of teeth, glossy toothpaste-white and decayed black. It no longer unsettles me; I've grown used to it. I like it when she smiles, and even more when it's me that's making her smile. She's like that, Anya; she's always smiling, always laughing. She never cries because there isn't enough of her to share.

Anya may not be able to make herself understood that well – at least, not in any language I can figure out – but she knows one song, and that's what will save my life tonight if someone happens to drop by and see me. See me spraying my message onto the wall, onto the world, onto the minds of the thousands of people who will see it on their way to another day of filing papers and making notes and emailing and holding endless, interminable meetings in anonymous grey rooms. Tomorrow morning, if all goes to plan, everyone will know who we are and what we can do. Once upon a time, they paid us to paint on walls. Now they shoot us.

I never set out to be subversive; or maybe I did, I don't know. I can barely even tell any more. But I know why I do this, all the same. You see, sometimes, when I'm out on the streets, I see these people. At first glance, you might mistake them for ordinary human beings. They have lungs and eyes and ears. They walk and breathe. If you listened hard enough, you might hear their hearts beating. But in reality, they're the grey people, the thin people – newspaper cutouts, robots programmed to live. To walk and talk. To smile and eat. To sit and roll over on command. Their minds are grey boxes, cabinets overflowing with meaningless little phrases and reminders. I want them to see what I see. A world without art is a world without life.

I finish the message at the bottom of the painting and stand back to admire it.

My contemplation is interrupted as a hum breaks through the still air, wavering but still clearly recognisable. Anya is humming, her voice quivering through the notes, a fly trapped in a jar and singing for its supper. The opening chords to our song.

Someone's coming, and they've seen us.

Fumbling, I scoop up the cans, trying to muffle the clatter of metal on metal, and bundle them into my rucksack. The flap is loose, open. No time to zip it. I curl my hand around Anya's and pull, dragging her behind me out of the dingy alley.

The sky is a yellowed bruise tonight, the world below it full of mirrors and shadowed planes. On postcards the city is sparking with electricity, flattened mosques against the fingernails of silver buildings rising above the smog. But from where I stand, on concrete paved with cans, crumpled paper and other waste, the slums are not just filth and odours, drugs and gunshots, or poor men. They are humanity. They were me, and they are you.

o0o

When I was eight years old, I found a key under my grandpa's mattress. The key was iron, and corroded with rust. I tried every lock in the house before I found one that worked – the rosewood cabinet in his study, the cabinet that he had always kept locked. I inserted the key. At first, I'd thought it wouldn't work. Then with a sound like shoes dragging over asphalt, it turned. Inside the drawer there was a pile of books – desecrated, torn, faded. I lifted the first book off the pile. It was a wreck, I saw that immediately. It must have belonged to my grandparents. Long ago, these kinds of insubordinate books had been burned, the brightness of their pages melting seamlessly into the leaping burgundy flames that consumed them. And yet this one had survived.

As I turned the pages, I could smell the must and decay on the yellowing parchment, and within the pages I saw dragons, unicorns and maidens and girls at windows, tigers in forests and soaring peaks of mountains, oceans, dreams.

From that moment, I knew what I wanted to do with my life. I used to be a writer, but from that moment on not a word spilled from my pen – only art, forbidden art, art that was viewed as a rebellion and a threat. A menace to society. Unless it was political, went the motto, it didn't count. But I didn't care about that. Hours upon hours I spent up in that study, ferreting out every book I could find. I soaked them all in. I taught myself the arts, copying the works of Da Vinci, Michelangelo, Bosch.

And that had brought me here.

o0o

"It's all right," Anya says softly into my ear. Her breath is warm and moist. She raises a finger, pointing it into the sky, a black pillar that blots the whole world out. "Here comes the sun."

She's wrong, again. It's evening, deepening into night, and the sun has burned itself down the horizon. But maybe she means something different. After all, you never can tell, with Anya, just how much she understands.

"You know, Anya," I tell her, "I think you might be right."

She smiles at me again, and squeezes my hand. Her fingers feel firm, warm, alive. I'm alive, too – for the first time in a long time.

And as we skulk hand in hand across the scars of the city, humanity pushes against me on every side, the tick-tick-ticking of over one million people radiating through my body, clawing desperately at my beating heart.