"There must be more than just the two of you," he said. "So far I've seen only the Brains and their giant women. What's happened to the rest of humanity?"

Paul rose and went to a wall with a large screen. He pressed a button, and Christian saw space with lots of stars.

"The humanity is there," said Paul. "Man has reached the stars"

Christian felt proud. After all, the human race was not really represented by the Brains. The stars… he dreamed a little, then remembered the reality.

"Do you know that the Brains have traveled into the past?" he said. "That they have come into the twentieth century and are murdering the best scientists of my time? Can the future be changed?"

Paul frowned. "That is a big question," he said, "even bigger than you think. I am not sure that I know the answer. Perhaps, it is a terrible mistake to try to change the past."

"But that's just what the Brains are doing!" said Christian. "Can you stop them?"

Perhaps," said Paul and rose from the table. "Let us go to the control room."

They came into a large round room with a large screen on the wall and a switchboard before it.

Paul said: "We can send the robots from here, and they will act as our eyes. We shall see on this screen all that they will see on the ground and underground."

He pressed some buttons on the switchboard, and the screen came to life.

The desert started moving before their eyes.

"Our robots could not help you before," said Barbara. "They can only see. As soon as we saw you, we came."

The robots approached the high wall near which Christian had been attacked by the ants.

"Why was this huge wall built?" asked Christian.

"It was an attempt to keep back the insects," said Paul. "Before people left this planet, these giant insects threatened them. They had mutated from small forms because of atomic radiation."

The robots came up to the lift and went underground. Christian saw the jungle in which he was hunted by the Brains.

"At first it was a cultivated hydroponic garden," said Paul, "but the Brains did not take care of it, and now it is wild."

The robots passed the jungle and moved along the corridors to the transmitter hall.

The time machine looked like a monster with many red lights. There were very few crystals in it now…

"It means," said Christian, "that they have gone into the past."

Paul nodded.

"Can you destroy this time machine?" asked Christian.

"Of course," answered Paul. "But what then?" You will remain here for ever - and the Brains will be destroying life in your time. Our problem is to reverse the situation."

He frowned.

"It is a problem which is not easy to solve," he said.

Paul asked Christian to tell them what the Brains were doing in his time, and he told them all he knew.

"It is clear," said Paul when Christian had finished, "that the Brains are trying to change the course of history, to remake the world and be its masters. If they do it, the result will be-"

"the end of civilization," finished Barbara. "No star colonies. Only the Brains and their giant women. It's awful!"

Paul nodded.

"We must do something to stop them," he said. "But what?"

He looked at the time machine on the screen.

"It is necessary to understand how this machine operates," he said. "I must concentrate… I will leave you with Barbara now, Captain. Later we'll speak again. Excuse me now."

Paul left them.

Barbara said:

"If you like, I will tell you about our history since your time."

"I'll be interested to hear that," said Christian. "I want to know more about you and Paul, and this ship and the star colonies."

"The time period is great," she began, "so I can only mention the main events that have brought us where we are today. First you must know that there was a war - the last terrible war, when atomic radiation flooded the planet, and whole continents were dead.

"This was the time of the insects: they mutated and became very big. The people who remained alive now divided into two camps: the true men and the Brains. It was impossible to live on the surface. We saw hope in leaving the planet and flying into space to look for other worlds. The Brains decided to go underground.

"We built our spaceships and flew to other planets, but found that it was quite impossible to live on both Venus and Mars.

"Try to imagine Earth as it was then, ruined and radioactive, and full of giant insects. The Brains had already gone underground and organized their colony there. Even at that time they were already beginning to degenerate. And you have seen the results of that wrong idea. Knowledge alone is worthless - there must be wisdom to control it.

"Finally we succeeded in building powerful spaceships which could fly far into space and reach stars.

"We flew into space and organized the first star colonies. Since that time we have organized many colonies. We have studied a lot of worlds and got a lot of knowledge.

"We wanted to help the Brains, but they rejected our help and did not want to co-operate. By our standards they were dangerous. So we set up a base on the moon to watch over them. You must understand that it is quite impossible for us to kill them…

"There are always two Watchers on this moon base. Our period of duty lasts twelve months."

Shocked, Christian exclaimed: "So there are only two of you? But of course, you can call for help."

Barbara smiled

"You have no idea, Captain, how great is the distance that separates us from the nearest colony. No. Paul and I must solve this problem alone."

Paul joined them.

"I have studied the time machine," he said, "and now I understand the principle on which it works. The black crystals are the main part of it. They appeared many thousand years ago as a result of the great atomic radiation of the Third World War. They are products of multi-dimentional hyperspace. The Brains discovered and studied them, and learnt to use them for moving through time. But they cannot make new crystals. So when we return all the Brains back into their time, I shall destroy their time machine and crystals, and they won't be able to travel through time and bother you."

"What shall we do now?" asked Christian.

"We shall go into your time and look for the Brains."

"All right, let's go!"

In their spaceship Paul and Barbara had a machine which looked like the robots, but could carry passengers. They got into it and started in the direction of the lift

In the underground jungle and corridors they met some giant women, who looked curiously at them, but no one tried to stop them. In the big hall the two robots were still standing near the time machine. Several Brains were looking down at them from the top of the machine.

Paul raised his voice.

"We are the Watchers. You must return to your cells and remain there till I call you."

One of the Brains raised his decapitator and fired at Paul. But nothing happened.

"There is no danger." said Paul. "I have just erected an invisible energy barrier around us. Nothing can pass through it."

Paul spoke to the Brains again. "You must go at once, before I lose patience," he said. The Brains got down from the top of the time machine and left the hall.

"Well," said Christian, "what shall we do now?"

"Our task is simple," answered Paul. "To bring back the Brains who are now in your time and then to destroy the time machine."

He turned to Barbara.

"I am afraid you must remain here, my dear," he said. "One of us must be here to guard the machine."

Barbara nodded.

Paul took a crystal out of the machine and fastened it to his belt. Then he took another crystal and fastened it to Christian's belt.