He had refused to commit himself one way or the other when Jan made his request, but he knew what his answer was going to be. Here was the opportunity for a most interesting experiment. It was a pity that he would never know the result; still, that happened often enough in scientific research, and he had initiated other programmes that would take decades to complete.

An earlier age would have regarded Professor Sullivan as an expensive luxury. His operations cost as much as a small war: indeed, he could be likened to a general conducting a perpetual campaign against an enemy who never relaxed. Professor Sullivan's enemy was the sea, and it fought him with weapons of cold and darkness -- and, above all, pressure. In his turn, he countered his adversary with intelligence and engineering skill. He had won many victories, but the sea was patient: it could wait. One day, Sullivan knew, he would make a mistake. At least he had the consolation of knowing that he could never drown. It would be far too quick for that.

As for Jan, he was now having second thoughts. The momentum of his original discovery had carried him thus far almost without effort. He had made his investigations, but had taken no active steps to turn his dream into reality. In a few days, however, he must make his choice. If Professor Sullivan agreed to cooperate, there was no way in which he could retreat. He must face the future he had chosen, with all its Implications.

Professor Sullivan was a brave and an intelligent man, but looking back on his career he was conscious of the fact that it had not brought him the sort of fame that sends a scientist's name safely down all the centuries. Here was a chance, totally unexpected and all the more attractive for that, of really establishing himself in the history books. It was not an ambition he would ever have admitted to anybody -- and, to do him justice, he would still have helped Jan even if his part in the plot remained forever secret.

What finally decided him was the thought that, If he neglected this incredible opportunity, he would never forgive himself. All the rest of his life would be spent in vain regrets -- and nothing could be worse than that.

"Dear Maia, This is going to be -- to put it mildly -- rather a surprise for you. When you get this letter, I shall no longer be on Earth. By that I don't mean that I shall have gone to the Moon, as many others have done. No: I shall be on my way to the home of the Overlords. I shall be the first man ever to leave the Solar System.

Sullivan's answer reached him a few hours later, and he knew that the die was cast. Slowly, because there was still plenty of time, he began to put his affairs in order.

(the letter began)

"I am giving this letter to the friend who is helping me: he will hold it until he knows that my plan has succeeded -- in its first phase, at least -- and that it is too late for the Overlords to interfere. I shall be so far away, and travelling at such a speed, that I doubt if any recall message can overtake me. Even if it could, it seems most unlikely that the ship would be able to put back to Earth. And I very much doubt if I'm all that important, anyway.

"First, let me explain what led to this. You know that I've always been interested in space flight, and have always felt frustrated because we've never been allowed to go to the other planets, or to learn anything about the civilization of the Overlords. If they had never intervened, we might have reached Mars and Venus by now. I admit that it is equally probable that we would have destroyed ourselves with cobalt bombs and the other global weapons the twentieth century was developing. Yet sometimes I wish we could have had a chance of standing on our own feet.

"Now I don't pretend to understand how that information reached us, or where it originated. Did someone read Rashaverak's mind? Even if they had, it's hardly likely that he would have known the reference number of his sun in one of our catalogues. It's a complete mystery, and I leave it to people like Rupert to solve -- if they can! I'm just content to take the information, and to act on it.

"Probably the Overlords have their reasons for keeping us in the nursery, and probably they are excellent reasons. But even if I knew what they were, I doubt if it would make much difference to my own feelings -- or my actions.

"Everything really began at that party of Rupert's. (He doesn't know about this, by the way, though he put me on the right track.) You remember that silly seance he arranged and how it ended when that girl -- I forget her name -- fainted? I'd asked what star the Overlords came from, and the reply was 'NGS 549672. I'd not expected any answer, and had treated the whole business as a joke until then. But when I realized that this was a number in a star catalogue, I decided to look into it. I found that the star was in the constellation Carina -- and one of the few facts that we do know about the Overlords is that they come from that direction.

"We know a lot now, through our observation of their departure, about the speed of the Overlord ships. They leave the Solar System under such tremendous accelerations that they approach the velocity of light in less than an hour. That means that the Overlords must possess some kind of propulsive system that acts equally on every atom of their ships, so that anything aboard won't be crushed instantly. I wonder why they employ such colossal accelerations, when they've got all space to play with and could take their time picking up speed? My theory is that they can somehow tap the energy fields round the stars, and so have to do their starting and stopping while they're fairly close to a sun. But that's all by the way…

"The important fact was that I knew how far they had to travel, and therefore how long the journey took.

"NGS 549672 is forty light-years from Earth. The Overlords ships reach more than ninety-nine per cent of the speed of light, so the trip must last forty years of our time. Our time: that's the crux of the matter.

"I have made calculations based on what we know about the stardrive, and using the firmly-established results of Relativity theory. From the viewpoint of the passengers on one of the Overlord ships, the journey to NGS 549672 will last not more than two months -- even though by Earth's reckoning forty years will have passed. I know this seems a paradox, and if it's any consolation it's puzzled the world's best brains ever since Einstein announced it.

"Now as you may have beard, strange things happen as one approaches the speed of light. Time itself begins to flow at a different rate -- to pass more slowly, so that what would be months on Earth would be no more than days on the ships of the Overlords. The effect is quite fundamental: it was discovered by the great Einstein more than a hundred years ago.

"Perhaps this example will show you the sort of thing that can happen, and will give you a clearer picture of the situation. If the Overlords send me straight back to Earth, I shall arrive home having aged only four months. But on Earth itself; eighty years will have passed. So you understand, Maia, that whatever happens, this is goodbye.

"I've terminated my studies and told the authorities that, for family reasons, I'm moving to Europe. Everything has been settled and there should be nothing for you to worry about.

"I have few ties binding me here, as you know well enough, so I can leave with a clear conscience. I've not told mother yet: she would get hysterical, and I couldn't face that. It's better this way. Though I've tried to make allowances, ever since father died -- oh, there's no point now in going into all that again!

"By this time, you may imagine that I'm crazy, since it seems impossible for anyone to get into one of the Overlord ships. But I've found a way. It doesn't happen very often, and after this it may never happen again, for I'm sure Karellen never makes the same mistake twice. Do you know the legend of the Wooden Horse, that got the Greek soldiers into Troy? But there's a story from the Old Testament that's an even closer parallel…"

"You'll certainly be much more comfortable than Jonah," said Sullivan. "There is no evidence that he was provided with electric light or sanitation. But you'll need a lot of provisions, and I see you're taking oxygen. Can you take enough for a two months' voyage in such a small space?". He stubbed his finger on the careful sketches which Jan had laid on the table. The microscope acted as a paper weight at one end, the skull of some improbable fish held down the other.

"I hope the oxygen isn't necessary," said Jan. 'We know that they can breathe our atmosphere, but they don't seem to like it very much and I might not be able to manage theirs at all. As for the supply situation, using narcosamine solves that. It's perfectly safe. When we're under way, I'll take a shot that will knock me out for six weeks, plus or minus a few days. I'll be nearly there by then. Actually, it wasn't the food and oxygen that was worrying me, so much as the boredom."

Professor Sullivan nodded thoughtfully. "Yes, narcosamine is safe enough, and can be calibrated fairly accurately. But mind you've got plenty of food handy -- you'll be ravening when you wake up, and as weak as a kitten. Suppose you starved to death because you hadn't the strength to use a can-opener?"

"I'd thought of that," said Jan, a little hurt. "I'll work up through sugar and chocolate in the usual way."

"Good: I'm glad to see that you've been into the problem thoroughly, and aren't treating it like some stunt you can back out of if you don't like the way it's going. It's your life you're playing with, but I'd hate to feel I was helping you to commit suicide."

"Luckily," continued Professor Sullivan, "the equipment you need is all fairly standard, and our shop can put it together in a few weeks. And if you decide to change your mind --"

He picked up the skull and lifted it absentmindedly in his bands. Jan grabbed the plan to prevent it rolling up.

"… I've considered all the risks I'm taking, and there seems to be no flaw in the plan. At the end of six weeks I'll emerge like any other stowaway and give myself up. By then -- in my time, remember -- the journey will be nearly over. We will be about to land on the world of the Overlords.

"Of course, what happens then is up to them. Probably I'll be sent home on the next ship -- but at least I can expect to see something. I've got a four millimetre camera and thousands of metres of film: it won't be my fault if I can't use it. Even at the worst, I'll have proved that man can't be kept in quarantine forever. I'll have created a precedent that will compel Karellen to take some action.

"I won't," said Jan.

"That, my dear Maia, is all I have to say. I know you won't miss me greatly: let's be honest and admit that we never had very strong ties, and now that you've married Rupert you'll be quite happy in your own private universe. At least, I hope so.

"Goodbye, then, and good luck. I shall look forward to meeting your grandchildren -- make sure that they know about me, won't you?

"Your affectionate brother Jan"