By Friday—three days after his arrival—Ed had learned three things about London.

The first was that the city ran on a system of informal economies that no guidebook could explain: you could buy anything if you knew who to ask, and you could find work if you knew where to look, and both required a kind of social navigation that Ed had never needed in San Francisco. The second was that London in June was capable of both brilliant sunshine and driving rain in the same hour, sometimes in the same minute, and that no one around him found this remarkable. The third was that he could not survive on nothing forever.

He needed money. He had paid Dorothy one pound for the first week, leaving him two pounds, and he had spent eleven shillings on food—bread, cheese, a pie from a shop near the British Museum—leaving him with a remainder that was not enough to think about. He needed an income, or he would be back on the street by the following Monday, and Dorothy Hargreaves was not a woman who made exceptions.

He had spent the first two days walking. Walking was free, and it was the only way he knew to learn a city—put one foot in front of the other and let the streets teach you their names, their rhythms, their grudging secrets. He had walked from Bloomsbury to the river and across the river to the South Bank, where the Festival of Britain was setting up its structures along the embankment: scaffolding and cranes and the skeleton of what would become the Royal Festival Hall, a building that was supposed to announce a new era to a world that was still rebuilding from the last one.

He had walked from Trafalgar Square to Piccadilly and from Piccadilly to Mayfair, where the buildings grew taller and the cars grew newer and the people grew colder, and then he had turned around and walked back, because there was nothing for him there.

He had walked to the British Museum on the third morning, and he had climbed the steps, and he had stood in the great entrance hall under the glass dome and felt, for the first time since the alley, something that was not panic.

The dome was high and blue and full of light. The floor was marble, polished to a shine that showed his reflection faintly—a ghost of a man in a borrowed sport coat. There were people everywhere—scholars and tourists and students and the particular species of London wanderer who used the museum as shelter from the weather or the city or their own lives. Ed stood in the middle of them and breathed, and he thought: I know this. I know what a library is. I know what a reading room is. I know what it feels like to be surrounded by books and to understand, at least a little, what they contain.

He went to the desk and asked about reading room privileges. The man behind the desk—old, spectacled, with the particular manner of a man who had been asking the same questions for forty years—told Ed that he needed a reader's ticket, which required two letters of recommendation from persons of standing, or a letter from his university, or a letter from his employer.

"I don't have any of those," Ed said.

"Then I cannot help you."

"Could I use the book stacks? Just to read?"

"The stacks are for researchers with proper credentials only."

Ed stood at the desk and looked at the old man's face and understood that there was no argument that would change this, no appeal that would work, no way to explain that he had been a mathematics graduate at Berkeley, that he had read Turing's paper in a seminar room with sunlight coming through the windows, that he knew what the library was for and how to use it without breaking anything.

"Could I speak to someone about the mathematics collection?" he said.

"The mathematics stacks are on the second floor, east wing. But you cannot access them without—"

"I'm sorry," Ed said. "I have to try."

He walked past the desk before the old man could call for security, and he climbed the stairs to the second floor, and he found the mathematics wing by following the signs that said PURE MATHEMATICS in black letters on white placards, and he pushed through the double doors into a corridor lined with glass cases full of journals and reprints and the faint, papery smell of knowledge that had accumulated for centuries.

The stacks were quiet. A few researchers bent over tables, writing in notebooks or flipping through the reprints stacked in front of them. A woman in a green cardigan was working through a problem set, her pencil scratching at the paper with the particular rhythm of concentration. Ed walked the stacks slowly, reading the spines: Proceedings of the Royal Society, The Quarterly Journal of Mathematics, Mathematische Annalen, Fundamenta Mathematicae.

He did not know what he was looking for. He was looking for something to hold onto, something that was his, something that would remind him of who he had been before the alley and the crack and the fall.

And then he found it.

It was in a glass case at the end of a narrow aisle, labeled in careful typescript: REPRINT SERIES, A. M. TURING, 1936–1947. And inside the case, on a bed of acid-free paper, was a pamphlet—a slim, pale blue booklet with the title in black type: On Computable Numbers, with an Application to the Entscheidungsproblem, by A. M. Turing, September 1936, reprinted 1947.

Ed stopped.

His hand went to his chest, to the pocket where the copy he had carried through the alley would have been—confiscated at the police station, inventoried, locked in a drawer somewhere in Marylebone. He had not seen it since. He had not thought about it in days, too busy surviving to remember the talisman.

But here it was. The original—the 1937 reprint, which was close enough, which was the same thing, which was the most important paper ever written about the nature of computation and the mind that produced it.

He looked around. The researchers were bent over their tables. The woman in the green cardigan was still working. No one was watching. Ed opened the case—he should not have done this, he knew he should not have done this, but his hands were already reaching—and he took the reprint out and held it.

The paper was thin, almost translucent, the blue cover soft with age and handling. He opened it to the first page and saw the words he had read in a seminar room in Berkeley in 1947, the words that had changed his life, the words that described a machine that did not exist yet and an idea that would change everything:

"1. Computing machines.

"1.1. Introduction.

"We may compare a man in the process of computing a real number to a machine which is only capable of a finite number of conditions..."

Ed sat down on the floor of the stacks. There was no chair nearby. There was no table. There was only the narrow aisle between the shelves and the soft light from the windows at the end of the corridor and the paper in his hands, and he read.

He read the way he had read it the first time—not quickly, not greedily, but carefully, line by line, the way you read something sacred. The definitions. The proofs. The extraordinary, impossible leap from a simple question—can a machine compute anything?—to a proof that described a universal machine, a single device that could compute anything that was computable, given the right instructions.

Turing had asked: what do we mean when we say something is computable? And he had answered: a number is computable if there exists a set of instructions that, followed by a machine, will produce that number. And then he had asked: what is a machine? And he had answered: a machine is anything that can be in one of a finite number of states, and that can move from state to state according to a table of rules.

It was the most elegant description of computation that had ever been written. It was also a description of the mind—not a metaphor, not an analogy, but the thing itself. If the mind was a computational system, then it could be studied with the same tools as computation. The brain was hardware. The mind was software. The distinction was Turing's, though he had not used those words, and it was a distinction that would take the world forty years to understand.

Ed sat on the floor of the stacks and read, and he thought about the man who had written this paper—Alan Turing, thirty-nine years old, living somewhere in England—and he thought about the fact that he was here, now, in this city, in this time, and he could not go home.

He thought about the fact that Turing would die in 1954. That there was nothing he could do about it. That the man who had described the nature of computation would be dead before he was forty-two, destroyed by a law that called his love a crime and his existence an offense.

He thought about the fact that he was holding a reprint of that paper in his hands, in a library in London, and that he was alive, and that Turing was alive, and that the distance between them was not infinite.

He thought about the fact that he was going to meet him.

He didn't know how he knew this. He didn't know how it was possible. But he sat on the floor of the mathematics stacks with the paper in his hands and the light coming through the windows at the end of the corridor, and he knew—with the same certainty that came from a proof, from a theorem, from a logical necessity—that he was going to meet Alan Turing, and that it was going to change everything.

He sat there until the light changed and the library began to empty, and a librarian came by and told him politely but firmly that the stacks were closing. He put the reprint back in its case—he was shaking when he did this, shaking with the effort of letting it go—and he walked out of the museum into the gray London evening, and he stood on the steps and looked at the city and thought: I know who I am.

I am a mathematician. I am a reader. I am a man who fell through a crack in the world and landed in a place where the man who changed my life is still alive.

I am going to find him.

I am going to tell him what I know.

I am going to do something that matters.

He walked back to Bloomsbury in the dark, and when he got to seventeen Grena Road, Dorothy Hargreaves was in the kitchen, and she had kept his dinner warm on the stove, and she looked at his face and said: "You look like a man who's seen a ghost."

"Not a ghost," Ed said. "Something better. Something that's still here."

She didn't ask. She never asked the questions whose answers she didn't want to know. But she set a place for him at the table, and she poured him tea, and she sat across from him and watched him eat, and when he had finished, she said: "Whatever it is, Mr. Morgan, I hope it doesn't get you killed."

"So do I," Ed said. "So do I."

He climbed the stairs to his room, and he lay on the lumpy mattress, and he looked at the ceiling, and he thought about Turing's paper and Turing's mind and Turing's life, and he thought: I cannot save him. I know that. I cannot stop what is going to happen.

But I can be there. I can know him. I can remember him when he is gone.

And maybe that is enough. Maybe that is what the crack in the alley was for.

He closed his eyes, and he slept, and he dreamed of machines.