“Yes — that’s a good idea,” said Julian. “We could hide somewhere till they come — and then set Timmy on them before they suspect anything!”

Almost before he had finished these words the light in the cave went out! Then a voice spoke out of the blackness.

“Keep still! One movement and I’ll shoot.”

George gasped. What was happening? Had the men come back unexpectedly? Oh, why hadn’t Timmy given them warning? She had been fondling his ears, so probably he had been unable to hear anything!

She held Timmy’s collar, afraid that he would fly at the man in the darkness and be shot. The voice spoke again.

“Will you or will you not give us your secret?”

“Not,” said Uncle Quentin in a low voice.

“You will have this whole island, and all your work blown up then, and yourself too and the others?”

“Yes! You can do what you like!” suddenly yelled George. “You’ll be blown up yourself too. You’ll never be able to get away in a boat — you’ll go on the rocks!”

The man in the darkness laughed. “We shall be safe,” he said. “Now keep at the back of the cave. I have you covered with my revolver.”

They all crouched at the back. Timmy growled, but George made him stop at once. She did not know if the men knew he was free or not.

Quiet footsteps passed across the cave in the darkness. George listened, straining her ears. Two pairs of footsteps! Both men were passing through the cave. She knew where they were going! They were going to escape by the undersea passage — and leave the island to be blown up behind them!

As soon as the footsteps had died away, George switched on her torch. “Father! Those men are escaping now, down the sea-tunnel. We must escape too — but not that way, My boat is on the shore. Let’s get there quickly and get away before there’s any explosion.”

“Yes, come along,” said her father. “But if only I could get up into my tower, I could stop any wicked plan of theirs! They mean to use the power there. I know — but if I could get up to the glass room, I could undo all their plans!”

“Oh do be quick then, Father!” cried George, getting in quite a panic now. “Save my island if you can!”

They all made their way through the cave up to the passage that led to the stone flight of steps from the little stone room. And there they had a shock!

The stone could not be opened from the inside! The men had altered the mechanism so that it could now only be opened from the outside.

In vain Uncle Quentin swung the lever to and fro. Nothing happened. The stone would not move.

“It’s only from outside it can be opened,” he said in despair. “We’re trapped!”