Nicolas has no choice but to ride on. The agony of the jarring, broken leg brings on a kind of temporary synaesthesia: he sees the colours of his pain in the corner of one eye, where it bursts like fireworks: gold, viridian and – for the most breathtakingly awful – purple. At one point, he has to stop to be sick, leaning over the saddlebow to keep the vomit off Tengri’s coat.

Walid approaches, looks at Nicolas’s bleary eyes and pallid face. ‘I have something that will help,’ he says. He reaches into a pouch on his belt and pulls out a tiny package wrapped in a piece of canvas. Inside is a pair of clay pipes and a dirty plug of something yellow. It looks like earwax. ‘Tryak.’ Opium.

The other men watch curiously as Walid prepares a pipe. It is a novelty to watch Nicolas get high. ‘The sahib cheated,’ Walid says, as he cuts off a piece of the drug and packs it down in the bowl of the pipe. ‘Are all such men dishonest?’

‘All men believe in fair play when it suits them,’ says Nicolas. ‘He’ll go home and say he was defending the honour of Her Majesty and that he defeated a Russian spy with cunning.’

Walid dismounts and opens his tinderbox. Shielding it from the wind, he strikes sparks onto its contents: a wad of waxed cotton, which he holds to the opium while Nicolas inhales. He gestures to Nicolas to keep the smoke in his lungs.

Nicolas has the feeling that a tiny dot of sunshine is spreading out through the nerves and ganglia in his stomach and lower back. It is a sensation of warmth and euphoria. ‘That must be why it’s called the solar plexus,’ he says. ‘Imagine that. Language is clever, isn’t it?’

‘Yes, master,’ Walid replies, and nods to the other men to let them know that the medicine is working. It’s time to take advantage of its analgesic properties. The men cluck at the horses and urge them forward with their heels.

Walid is concerned that Nicolas is too relaxed to ride properly. He’s sitting slumped in his saddle, with his eyes drooping, a glazed half-smile of drugged contentment on his lips. But Tengri senses his delicate condition and picks her way carefully along the path.

The men ride uphill all morning, through a narrowing defile that rises to the top of a ridge. Nicolas is the last to reach the crest. From here, a vast valley of grey rock is spread before them under the sapphire sky. In the sunlight, the ice on the distant glaciers acquires tints of blue and green. ‘It’s heaven up here,’ says Nicolas. ‘Maybe time for another pipe?’

‘Not now,’ says Walid, laughingly, admonishing Nicolas with his finger. ‘Not while you are in heaven. Only when you are suffering in hell!’ He kicks the flanks of his horse and heads over to the other guides to repeat his bon mot.