Thomas points, and as Charlie follows his finger he sees for the first time a smear of grey in the air up ahead. Not the dark plume of a fire, nor the clean contours of a storm cloud, but more like a fog, rising out of the ground, wet and stubborn, resistant to the winds. Within a minute, the landscape around them begins to be covered by a film of dark scum. Ahead lies the city: a hazy, dark sprawl from which grow the slender spires of factory smokestacks, their outlines cleaner, sharper than anything closer to the ground. After another minute the first houses start, grime-covered brick and narrow courtyards, washing lines full of linen more grey than white. Soon the Smoke outside the window becomes impossible to ignore: it tints their vision and saps the strength out of the sun. The train has slowed to walking pace and London seems everywhere, boxing them in in the narrow chasm of its streets. Something takes hold of Charlie, an emotion halfway between fear and spite. He wants to return to Oxford. And also: put the match to this city, see it burn. He is about to tell Thomas when Renfrew gets up, walks to the front of the carriage in his deliberate step. The air above his head is oddly hazy. The Smoke has long come inside, Charlie realises, has sniffed out cracks in the windows and doors and risen through the undercarriage, seeped into their clothes, their skin, their lungs.

“Some of you can feel it already,” Renfrew begins. “The Smoke. It’s making you feel — unusual. Afraid. Aggressive. Frivolous. Vain. Your thinking is beginning to be clouded; you dwell on things. The outside world is no longer separate from you but is beginning to insinuate itself into your being. You’re feeling small, insignificant, malleable, but are ready to fight anyone who dares to say so; your little store of prudence is eaten away as though by rats. Temptation presses herself on you — to steal, to cheat, to run away. All we have taught you—all—is put under pressure. It is as though someone has run off with your coat. You stand in your shirtsleeves, and the day is cold. And this is here, in a closed train compartment, a mile yet to the station. Outside, in the centre, amongst the people of London, it will be a hundred times more intense. Some of you may feel like you must succumb. I have one word for you.”

He pauses, fixes on their faces.

“Don’t.”

The word comes down like a blade. Even the other teachers seem startled.