‘Michael Baron – have you seen him? I’ve just followed him from the lighthouse. He must have come down here. Mags – he’s the saboteur!’

She blinked. ‘You’re insane, Petra,’ she said, after a stunned pause.

‘Mags! I’m serious – remember the wire cutters he had when he helped free you from the barbed wire? I just watched him use those to cut the telephone line to the Castle!’

‘What?Why would he want to do that?’

‘I don’t know, Mags! To cut off the line to the Admiralty, I suppose – to buy time if German U-boats or landing craft are spotted from the lighthouse? It could be lots of different things, but it’s not exactly going to be for a nice reason, is it?’ Then something else occurred to me – a flash of light. ‘MB!’ I said.

‘What?’

‘Look!’ I found the scrap of paper in my coat pocket and thrust it in front of her face. ‘This is what Spooky Joe was writing down that morning when we were spying on him.’

Mags frowned. ‘I thought I’d lost—’

‘Well, I found it again,’ I interrupted. ‘I don’t know what all the numbers mean yet but look, here at the top – MB – Michael Baron!’

‘MB could stand for anything, you idiot,’ Mags spat. ‘It could stand for . . . motor boat or moon beam or . . . mountain bear!’

It was my turn to blink at her. ‘Why would anyone be making secret notes about mountain bears, Mags? At first I thought it might even have been you – Magda Bernadette – but then when I saw Michael—’

‘You thought what?’

‘Please, Mags, listen! It must be Michael – it all fits together, don’t you see? He’s the one behind the sabotage, Mags. I bet it was him that cut the other telephone lines and set the village hall on fire too!’

‘You’re wrong, Petra,’ Mags said, her voice trembling with rage.

Why wouldn’t she listen to me?

‘I’ve got to go straight to the police station,’ I said. ‘Come with me, Mags – please.’

She shook her head. She looked frightened, furious. ‘You’re being ridiculous,’ she said. ‘I don’t believe a word of it. You need to stay here where you are safe, Petra. The whole point of coming to the cave was to protect us from being sent away. If you go to the police . . .’

‘This is more important than that, Mags. Can’t you see? This is about the war, the invasion!’

I saw that her hands were shaking. What was wrong with her? What had happened to my sister – the fearless girl who fought the bullies, the hero who had sailed over to Dunkirk and saved the lives of our stranded soldiers? Through the eyeholes in the rock behind her, I could see dark storm clouds swelling and gathering fast. My sister’s eyes were just as angry.