A touch of paranoia would have served Dennison well; but he was lacking in that vital characteristic of inventors. And he didn't even realize the full extent of his carelessness until a bullet, fired from a silenced weapon, chipped a granite wall not three inches from his head.

Then he knew. But by then it was too late.

Charles Dennison had been left a more than adequate income by his father. He had gone to Harvard, served a hitch in the Navy, then continued his education at M.I.T. Since the age of thirty-two, he had been engaged in private research, working in his own small laboratory in Riverdale, New York. Plant biology was his field. He published several noteworthy papers, and sold a new insecticide to a development corporation. The royalties helped him to expand his facilities.

Dennison enjoyed working alone. It suited his temperament, which was austere but not unfriendly. Two or three times a year, he would come to New York, see some plays and movies, and do a little serious drinking. He would then return gratefully to his seclusion. He was a bachelor and seemed destined to remain that way.

Not long after his fortieth birthday, Dennison stumbled across an intriguing clue which led him into a different branch of biology. He pursued his clue, developed it, extended it slowly into a hypothesis. After three more years, a lucky accident put the final proofs into his hands.

He had invented a most effective longevity drug. It was not proof against violence; aside from that, however, it could fairly be called an immortality serum.

Now was the time for caution. But years of seclusion had made Dennison unwary of people and their motives. He was more or less heedless of the world around him; it never occurred to him that the world was not equally heedless of him.

He thought only about his serum. It was valuable and patentable. But was it the sort of thing that should be revealed? Was the world ready for an immortality drug?

He had never enjoyed speculation of this sort. But since the atom bomb, many scientists had been forced to look at the ethics of their profession. Dennison looked at his and decided that immortality was inevitable.

Mankind had, throughout its existence, poked and probed into the recesses of nature, trying to figure out how things worked. If one man didn't discover fire, or the use of the lever, or gunpowder, or the atom bomb, or immortality, another would. Man willed to know all nature's secrets, and there was no way of keeping them hidden.

Armed with this bleak but comforting philosophy, Dennison packed his formulas and proofs into a briefcase, slipped a two-ounce bottle of the product into a jacket pocket, and left his Riverdale laboratory. It was already evening. He planned to spend the night in a good midtown hotel, see a movie, and proceed to the Patent Office in Washington the following day.

On the subway, Dennison was absorbed in a newspaper. He was barely conscious of the men sitting on either side of him. He became aware of them only when the man on his right poked him firmly in the ribs.

Dennison glanced over and saw the snub nose of a small automatic, concealed from the rest of the car by a newspaper, resting against his side.