Mags’s face was shining, but I felt as I always felt when I stood here in the cave. I felt eerie. The two windows at the front made me feel as if I were inside a giant skull, peering out through the hollow eye sockets.

‘Just look at the Wyrm,’ I muttered, staring out through the left eyehole. I looked down, and swallowed.

The tide was coming in, the sands were shifting, and the yellow-grey Wyrm was twisting about hungrily beneath the shallow waves.

‘It must be tricky, that sandbank,’ Michael said. ‘If you’re manoeuvring a big ship, or a submarine or something like that.’

‘It’s fine if you know what you’re doing,’ Mags said with confidence. This was her favourite subject. ‘And the lighthouse gives you the perfect bearing to steer safely around it into Dragon Bay or Stonegate harbour.’ She looked at Michael. ‘I can show you if you like.’

He nodded, keen as mustard. ‘That would be wonderful.’

Down at Dragon Bay, Mags led us straight to the line of upturned rowing boats that were pulled up high on to the dry sand, out of reach of the tide.

‘Aren’t we going to take the motor boat?’ I asked. Pa had salvaged the rusty old wreck from the scrapyard as a project for Mags. When we first got it, it just about managed to stay afloat, but the engine had needed rebuilding. Mags had christened her Faith, and she was moored in the harbour.

‘No, I haven’t quite got the engine running yet,’ Mags replied.

‘But you’ve been working on her for months, haven’t you?’

I might have been mistaken, but I thought Mags flashed a look at Michael then.

‘Of course I have. But there’s so much that needs doing before she’s safe and seaworthy. Come on – we’ll take Edward.’

Edward, or rather King Edward, was the family rowing boat we had used for the crabbing competition – a lovely old thing, painted red, blue and white. I used to think it was odd that we had a boat named after a potato, but it had been Pa’s since he was a boy (when King Edward VII was on the throne). Pa had passed it on to Mags, who had always tended it lovingly. If Mags went missing on a summer’s day, she could usually be found down here on the beach, sanding, repairing and repainting King Edward, her sleeves rolled up to her elbows, flecks of paint in her messy hair. My sister had always smelt of yacht varnish and the sea.