Christian was back in London. He was speaking to the Prime Minister and the head of Military Intelligence. He had just finished telling them about Paul's meeting with the Brains, when the telephone rang. The Prime Minister took up the receiver and listened. Then he put down the receiver.

"Operation Finish," he said, "is starting now."

The Old Man jumped up.

"Have they found the Brains?" he asked

"Yes," said the Prime Minister, walking to the door. "We must go to the Headquarters of the Operation. Captain Christian, you may go with us."

They hurried along the corridor and up a staircase. They entered a room and went up to a large table in the middle. There were several officers of high rank near the table. On the table there was a large map, and two girls moved coloured pieces of plastic on it, as the radio-telephone announced coordinates.

"Good!" exclaimed the Prime Minister, rubbing his hands. "Now we can finish this business without Mr. Paul. The enemy has seized the Imperial Technical Institute, and they are all there now. And that is just what we want."

He looked at Christian.

"This plan has been ready for some time," he said to the captain, "but we had to wait for the right moment. A jet plane loaded with an atom bomb is standing not far from the Institute. We are now evacuating people from the area around the Institute. As soon as all the people are evacuated - we shall strike."

The radio-telephone brought reports every minute.

"Section E-one cleared. Transport moves along route North-East."

"Section A-ten cleared. Section E-three cleared. Section…"

Christian thought that perhaps the Brains would leave the Institute long before the end of the evacuation. The Prime Minister watched the map and every now and then looked at the clock on the wall

The radio-telephone continued bringing new reports.

"Section M-eleven cleared. Transport for the last section is ready."

Tension was growing in the operation room. It seemed that the time had stopped. Then -

"Final clearance," announced the radio-telephone. "Repeat final clearance."

The Prime Minister said: "Connect me with the airfield."

"Waiting, sir," said the operator and gave him the phone.

"Commodor Bateson?" said the Prime Minister into the phone. "This is the Prime Minister speaking. Begin the last phase of Operation Finish."

He sat down, smiling nervously.

"Well, gentlemen, we can only wait now. The flight will take seven minutes."

They were the longest seven minutes in Christian's life. Now, he hoped, was the end of the Brains.

The pilot's voice sounded on the radio: "Approaching target area."

A pause. Then -

"Bomb is thrown!"

The Prime Minister stood up. He was very excited. Second after second passed. Then the radio spoke again.

"It's the operator speaking. The bomb did not explode…»

The Prime Minister's face turned grey. It seemed that he was ready to burst into tears.

Christian left the operation room. He was very sad. Once again the Brains had shown that they were clever. People could do nothing against them. The only hope was Paul.

The Brain's answering attack came very soon.

In New York, the Security Council of the United Nations was in session. The conference hall was full of delegates. They were discussing the latest events. The Secretary-General stood up.

"Senor Mendez has the floor," he announced.

Mendez, a big man with dark hair and moustache, rose and looked around the hall at the faces of outstanding politicians. Bright sunlight came in through the large windows. Outside the sky was clear and blue.

"We must attack again," said Mendez, "but not from the sky - from the ground…"

It seemed to him that it was very hot in the hall. Or was it only his imagination? He touched his collar.

"…a big attack from the ground…" he continued.

It was really hot. The temperature in the hall was quickly rising.

"…use heavy tanks…"

Mendez could not speak any longer. The people in the hall had also noticed the rise of the temperature. They were jumping up from their seats and running to the doors. The glass walls began to melt. A woman screamed: her dress suddenly began to burn…

In a few minutes the great building of the United Nations was burning. The New York firemen could not put out the fire because the water in their pipes was turning into steam. Nobody could do anything… and many of the world's outstanding statesmen perished in the great fire.

The tanks were moving forward. There were three rows of them, the most modern heavy tanks with powerful guns. They were making a circle around the buildings of the Imperial Technical Institute.

The commander was watching them through field-glasses from his observation post. He was a thin, grey-haired veteran, very sure of himself and very proud of his tanks.

The commander smiled to himself, remembering the bomb which had not exploded. Such things sometimes happened to bombs. But he was sure that nothing could stop his tanks. He continued watching them through his field-glasses. They were already near the Institute. Another second or two - and he was going to give the signal to open fire.

Suddenly something was wrong. The first row of tanks stopped. They began to change their shape. They were melting, like ice.

The commander seized his microphone. "Open fire!"

The second and third rows of tanks stopped, too. The commander saw through his field-glasses that they were melting. They were melting very quickly In a few minutes there were no more tanks. There were great pools of melted metal in their place.

Captain Christian threw the end of his cigar into the Thames and stepped down into the cabin. Paul was still working on his apparatus. It was a small box with wires, and it was difficult for Christian to believe that it could be an effective weapon against the Brains.

"Have you read the report about the tragedy in New York?" Christian asked.

Paul nodded.

"Do you understand how the Brains did it?"

"They have modernized their decapitation weapon. They release the energy of the air by remote control."

"Can your energy barrier hold out against that?"

"Yes."

"Then you must teach our scientists to erect this energy barrier. The Brains may attack with their new weapon again."

Paul shook his head.

"No, Captain, I will not interfere. The people of your time are not ready for such a weapon. But my apparatus is nearly ready - the Brains will not bother you any longer."

Paul connected his apparatus to the energy source. A high-pitched sound came from the little box. It was getting higher and higher. It was resounding in the captain's head. Christian felt that he wanted to scream. The sound was getting still higher… higher… then Christian could not hear it any more. But the needle moving across the dial on the machine told him that the sound was still coming from the little box.

Paul explained: "I have built a high-frequency oscillator. It broadcasts sound waves which cannot be heard by a human ear. The sound is continuous and will last indefinite time."

"Well?"

"The structure of the Brains' ear is different from yours. -They will hear the sound, and it is impossible for them to stop it. From this moment they will always have this noise in their heads. I don't think they will be able to stand it for a long time.. and to escape from this noise, they must return to the future."

Christian shuddered.

"For me a few seconds were enough!" he said.

"Quite so," said Paul. "So the Brains will have to return to their time. When they return, I shall destroy their time machine, and you can live in peace."