The message reached Amiens a day later. Captain Ellis took it to General Bruce. ‘We could walk into Bray and take the town back with hardly a shot, they say?’

‘Yes, sir.’

‘When the time comes we may do just that. If we can make a surprise attack then that’s what we’ll do. But it’s best if we can take Bray with no shells falling on the town and killing the local people still there. I hear we had an accident when we retreated.’ The captain gave a silent nod.

They were in the general’s office in Amiens. It had once been the grand house of the mayor. Now its fine windows were criss-crossed with blast tape and the shining floors scuffed with a hundred army boots.

The general walked from behind his desk and crossed to the door. He stood with his back to it. He looked across at Captain Ellis and winked. ‘Who is our spy in Bray?’ he asked in a clear voice.

The captain replied carefully, ‘If the Germans ever found out they’d execute him.’

‘True. But you can tell me.’

Captain Ellis took a deep breath and answered in a loud voice, ‘The German spy in Bray is a German officer. He wants to see an end to the war so he is passing messages to us. He takes them to the airfield and one of their pilots drops a message into our garden here.’

On the other side of the door a man with hawk eyes listened then hurried away on tiptoe. His left hand was silver.

Captain Ellis listened to the fading footsteps. ‘Do you think our friend Grimm heard that?’ he asked softly.

‘I’m sure he did. He will pass on the message through his own spy network and the Germans will chase their own tails looking for traitors in their army and air force. Our White Lady spies will be safe.’

Captain Ellis spoke softly. ‘We can’t let him go on spying on us for much longer, sir. He may be sending real secrets along with the gobbledygook we are giving him.’

General Bruce spread his hands. ‘We know he’s a traitor. But he’s useful. We can feed the enemy nonsense. When the time is right – when he stops being useful – we’ll arrest him and execute him. But we need evidence.’

‘The girl Aimee heard him pass on the formula for the DM gas, sir,’ Captain Ellis reminded him.

‘The false formula we invented to catch the spy will confuse the German chemists for a few weeks.’ General Bruce chuckled. ‘But we just have the girl’s word that Sergeant Grimm gave it down the phone.’

‘I believe her,’ Ellis said. ‘And I think he gave himself away when he tried to throw her out of the balloon.’

‘If he did,’ the general said.

‘I believe her,’ the captain repeated stubbornly.