At eleven o’clock she got up to go. But then she stopped at the door. She looked back at me.

“ ‘Tell me, Helen,’ she said. ‘Have you ever heard someone whistling in the dead of night?’

“ ‘Never,’ said I.

“ ‘You’re sure you don’t whistle in your sleep?’ she asked.

“ ‘No, I don’t,’ I said. ‘Why do you ask?’

“ ‘I ask because I keep hearing a whistle. I hear it about three o’clock every morning. I am a light sleeper. The whistle always wakes me up. I cannot tell where the whistle is coming from. Maybe it’s coming from the next room. Maybe it’s coming from outside. But I just thought I would ask if you had heard it.’

“ ‘No,’ I said. ‘I have not. Maybe it is those gypsies camping out there.’

“ ‘That must be it,’ she said. Then she said good night. I heard her go down the hall. Then I heard her turn the key in the door.”

“Hmm,” said Holmes. “Did you always lock yourselves in at night?”

“Always. We were afraid of the cheetah and the baboon.”

“Quite so. Please go on with your story.”

“I could not sleep that night. You will remember that my sister and I were twins. They say twins can feel things that are happening to each other.

“It was a wild night. The wind was blowing hard. The rain beat against the window.

“All of a sudden I heard a scream. It was my sister’s voice! I jumped up from my bed. I ran into the hall. It was then that I heard a low whistle! A few seconds later I heard a clanging sound.

“I ran to my sister’s door. It started to open. My sister came to the door. Her face was white with fear.