There was a rustling of papers again and Aimee knew she had discovered the traitor. She also knew that Sergeant Grimm might kill to keep his treason secret. If she walked into the room she could save the British and French so much trouble. And she also might die. Grimm was speaking quickly again. ‘They call it diphenylamine chlorarsine. I’ll spell it for you then give you the formula for how to make it.’

Sergeant Grimm gave his partner the details. Then he explained how it would be used. ‘The men in the trenches will put canisters of this DM gas on the edge of their own trenches. They’ll light a fuse and the wind will carry the cloud across no man’s land. When the Germans are coughing and vomiting the British will attack. They’ll give their own men special gas masks so they don’t suffer.’

Aimee knew she should have stopped him. She couldn’t decide... until the dust decided for her. She sneezed. The man with the silver hand muttered something in German and put down the telephone. Aimee made herself busy with packing the books into a box. Sergeant Grimm’s shadow turned the dark room into a tomb. He filled the dusty air with the scent of tobacco and sweat. She looked up at him and gave a warm smile. ‘Bonjour, Monsieur,’ she said.

‘Do you speak English, girl?’ the man asked and the fingers on his good hand rippled like a pianist’s. Aimee shivered.

‘Pardon?’ she said.

Grimm smiled and said, ‘That’s good. If you spoke English I’d have to kill you.’ The words were spoken in a soft and friendly voice.

‘Pardon?’

‘Get out.’

‘Pardon?’

He waved his silver hand at the door. Aimee pointed at the books and made a mime of packing them. The man shook his head and pointed at the door from the cupboard. She shrugged and walked past him into the office and towards the door into the hall. Silver Hand spoke soft as a butterfly’s wing. ‘You dropped some paper,’ he said in English.

Aimee almost stopped and turned to look for it. She remembered she wasn’t supposed to understand English and carried on. But that half stop was enough. Silver Hand knew. And Aimee knew he meant what he said. To keep his deadly secret safe he would have to kill her. And kill her soon.

23 March 1918: Peronne

The doctor with the spectacles and the worried face was called Weger. Marius soon learned that the doctor’s miserable appearance hid a sharp mind that loved to joke. The boy followed him through the hospital tents, weary as a fox at the end of a hunt. Doctor Weger never seemed to get tired. At night they moved, always west, chasing the setting sun.

‘I sleep as we travel,’ the doctor said.