Soundlessly, the optic started turning. It looked like a huge lampshade or drum with alternating panels of glass and wood – a giant zoetrope. As it rotated smoothly around the lamp, it gave the effect of the light flashing.

Pa pulled down the switch to light up the lantern and the beam of light shot out into the fog. We both watched the slow-flashing light as it attempted to penetrate the misty air. Each beam looked almost solid, I thought – a blinding-white rod of metal, a spear for a Greek god . . . Then there was a heavy click. I blinked, and the light had gone. The lantern seemed to glow for a moment – electric-orange, then red – and then the world was colourless once more.

Pa stood still for a while, his ocean-blue eyes fixed forward, as if in a trance. With the fading of the lamp, something seemed to have faded in him too. I stared out of the window, trying to catch a glimpse of the ship lost in the sea fret, but the world below was nothing but a smoky swamp.

Then, from somewhere out in the Channel, we heard the sound of a distant foghorn, and I felt an unexpected spasm of panic: Are we helping the right ship? Pa knew his job, of course; he knew his signals – but fear of the U-boat lurked just beneath the surface of my thoughts. What if the enemy had other vessels out there too, hiding in the fog?

The sound of the foghorn brought Pa out of his trance. He gave me a distracted little smile, kissed me on my forehead, then went back down the stairs. There’s something on his mind, I thought. He is very worried about something. Something important. I wondered if it was connected to that envelope he had just put in his pocket.

Silently, I added this to the list of puzzles in my head: the hidden photograph, the dark shape in Dragon Bay, Kipper’s accusation, my sister’s weird behaviour . . . And now Pa. A whole shoal of secrets, writhing in my mind like fish in a net.

The fog did not lift all that day, and it was still there the next morning, cloaking the cliffs in stillness and secrecy. Our Castle was a castle lost in the clouds.

The chill seeped into my bones as I tiptoed from the front door of the cottage and out into the pearl-grey gloom.

I was following Mutti.