The mass spectrometer came back from the dead on Wednesday, with a new circuit board and a faint smell of burnt dust, and by Thursday night Sienna had the whole basement laboratory to herself.

She had arranged it that way. She had told Dr. Pryce that a backlog of overdue assays required her to stay late, which was true, and she had told Nell she was going home early with a headache, which was not. By half past eight the corridors of Marenna General had emptied into the long hum of the night shift. The vending machine on level two was out of coffee. The pipes ticked. In the narrow room under the stairs, with the door closed and a sheet of brown paper taped over the little square window, she set out her samples in a neat row on the steel bench and looked at them with the particular, tender dread one feels for a patient who has not yet been told.

There were eleven of them.

She had collected them over four days, with a sterile swab, a pair of gloves, and a notebook in which she wrote down everything, the time, the place, who had been present. A dab from the inside rim of the reserve champagne bottle that Dario had ordered up for the wedding, which had sat unopened in the cellar rack. A scraping from the wax at its neck. Milk from the jug in the east kitchen, and tea from the pot, and water from three taps, and a spoonful of the coarse gray salt that Mrs. Venn kept in a pewter cellar by the range. A pinch of dust from the shelf in the library where the blue leather box had been locked away. And, in two small sealed tubes, side by side at the end of the row, the careful swabs she had taken on Tuesday evening from the inside of the Bellandi wedding flutes.

He had given her the key himself.

She had not asked him directly. She had said, over breakfast, in the voice she used when she meant to sound idle, that she would like to look at the crystal Dario had given them, and Matteo had set down his cup, and drawn a small brass key from his waistcoat pocket, and laid it beside her saucer. He had not asked why. He had only said, "The library, left of the window. Take your time. It's yours." Then he had gone out into the day with Marco, and she had sat for a long moment with the key warm in her fist.

She turned the tubes in the light. Two flutes, two swabs, a thin pale smear on each. They looked like nothing at all.

She pulled on fresh gloves, loaded the first of the vials into the rack, and began.

The broad screen took forty minutes. She sat on the stool with her chin in her hand and watched the numbers crawl up the monitor, line by line, and made herself not hope for anything. In her experience, hope was the great contaminant. It slipped into an assay the way a draft slips under a door, and it made you see peaks where there were only ripples. The reserve bottle: clean. The milk: clean. The tea, the water, the salt, which she tested twice out of a sheer, irrational tenderness: clean. The library dust gave a faint ghost of a household cleaning agent, a lemony polish, and nothing else.

It was a quarter past ten when she ran the first flute.

She did not expect anything. She had loaded it with the faint, flat resignation of someone who has checked every box and found it empty. She leaned back against the bench and closed her eyes for a moment, and listened to the instrument hum. When she opened them, the screen had changed.

She sat up.

A peak. A small one, low and narrow, far to the left of the main trace, in a region where there should have been nothing but background noise. She leaned in. It was not a toxin. She knew that at once, with the quick, sure part of her mind that had been trained for six years to recognize the ugly ones. It was too tidy, too quiet, and its shape was wrong. It looked like a quiet organic salt of some kind, the sort of thing you might find in a pharmaceutical stabilizer, or a lubricant, or the lining of an old glass.

She ran the second flute. The same peak, the same shoulder, a hair higher.

She ran both again, and then a blank, and then a standard, and then both again. She worked with a slow, mounting, thrilling dread, her hands moving by themselves over the instrument while the rest of her mind stood a little apart and watched. The peak did not move. It sat there on the screen.

A clean flute should have shown nothing.

She pushed back the stool and stood up. She walked the length of the little room and back, with her gloved hands pressed against her mouth, and looked at the trace on the screen from three different angles, as if it might turn into something else if she approached it from the right direction.

It was not the poison. She was nearly certain of that. She had the poison by heart. She had carried its signature in her mind for twelve months, the narrow panel, the rapid onset, the metallic bloom at the back of the tongue, and this was not it. It had none of the right masses. It could not stop a heart. It could not stop a mouse.

But it had been waiting.

She went back to the bench, sat down, and drew her notebook toward her. She made herself write it in order, one line after another, the way she would have written up any other puzzling result, because a thing written in order is a thing you can bear.

One. A compound is present on the interior rim of both flutes, in the etched band of leaves, concentrated in the grooves where a cloth cannot reach. Not a cleaning agent. Not a polish. It does not match the library dust.

Two. By itself, it is inert. It does nothing. It would not harm a child.

She stopped. She looked at the word inert, and at the long grim shadow behind it, and made herself continue.

Three. I know a family of compounds that sit quietly in this form, and wake up when something else arrives. Acid, usually. Or a second agent. Something that finds the sleeping one, and binds to it, and changes it.

She looked up from the page at the row of vials. Her eyes passed over the reserve bottle, and the swab from the wax at its neck, and stopped.

Champagne is acid. It always has been. A dry, cold, sparkling acid, poured into a glass.

She closed her eyes. She saw it, in a single, complete, sickening flash: a silver bucket sweating on a sideboard, a white ribbon, a bottle tilted in melting ice. A tall crystal flute, a spiral of facets up the stem, a band of leaves at the rim. A man's hand lifting the bottle. A thin gold stream of wine, falling, rising, touching the etched band, meeting something that had waited for it for a hundred and forty years or for six days.

And nothing in the glass. Nothing in the bottle. Nothing in any cup or plate she could have tested, if she had been testing, which she had not. The wine was only wine. The glass was only glass. It was the meeting that killed.

The poison had been made at the last. It had been made in her hand.

She sat motionless, with her pen resting against the page and a very cold sweat gathering along her spine. It was so elegant. That was the part she could hardly bear. It was elegant as a well-proved theorem. Nothing could be found on the food, in the bottle, in the cellar, in the cook's apron or the maid's tray, because nothing had been there. Every part was innocent. Every part could be tested and found clean, by a woman with a kit in her bag and a lifetime's caution. It was only the combination that was lethal, and the combination existed for the length of a sip.

She did not know who. She did not know how. But she knew now that whoever had done it had read the same books she had, and gone one chapter further, and had possessed the one thing no common murderer would have had: a long and patient and intimate knowledge of how a glass behaves.

Her eyes went again to the tube in the rack. Dario's voice came into her head, warm and rich as a cello. I took the liberty of having them cleaned. I thought it only fitting.

Cleaned. By whom?

She wrote it down. She did not underline it. She only wrote the question, and the date, and drew a small, careful box around the word cleaned, as one draws a box around a place on a map where one has seen a movement in the trees.

At ten forty, her phone buzzed on the bench.

She almost let it ring. She was deep in the arithmetic of the thing, working out which acid, which concentration, which temperature, and her hands were stiff and cold. But the screen glowed in the corner of her eye, and she turned it over without really looking.

I'm outside. Please don't hurry. I'll wait in the car.

She read it twice. It was from the number she alone possessed, and it was signed with nothing, and it was so entirely and carefully polite that for a moment she could not think what to feel about it. She had not asked him to come. She had told Mrs. Venn she would be late and might take a cab. But the message sat on her screen with its small, patient courtesy, and she heard in it the faint echo of a man standing outside a locked door at dawn, listening for a bolt.

She typed, then deleted, then typed again.

Clause four. Not inside the building.

The reply came back in less than a minute. Understood. May I wait at the end of the corridor, where the guard can see me? I'll stay outside the lab. I won't speak to you until you come out.

She stared at the screen. It was against every principle of her working life to let a man wait for her in a hospital basement at eleven at night, as though she were a woman in a novel. It was also, she thought, a reasonable request, framed as a request, by a man who had every right to simply walk in.

She typed: The bench by the stairs. Not the lab. And put the phone face down, and returned to her columns with a resolute, slightly flushed concentration, and did not think about it again.

It was a quarter to one when she finally stood up.

Her back ached. Her eyes burned. The notebook on the bench held eleven pages of figures, and a diagram she did not like, and a list of three questions, in order, with the date. She had done everything that could be done tonight. She packed the two tubes into a lockbox, sealed it, wrote her initials across the seal, and stored it in the bottom of the cold cabinet behind a row of boxes marked EXPIRED. She stripped off her gloves, washed her hands twice, and took her cardigan from the back of the stool. She turned off the lights. She opened the door.

The corridor of the lower basement was very long, and very cold, and lit by a single flickering tube. The pipes ran along the ceiling like the ribs of some enormous sleeping animal. At the far end, where the corridor met the stairs, a security guard had left his stool and his thermos and gone off on his rounds. And under the one good light, on a wooden bench against the whitewashed wall, with his coat folded neatly across his knee, sat Matteo.

She stopped in the doorway.

He had not seen her. He sat with his elbows on his knees and his hands loosely linked, staring at the floor between his shoes, with the stillness of a man who has been waiting a very long time and has stopped noticing that he is waiting. His hair had fallen forward over his forehead. His tie was gone. The pale light of the tube made him look gray and thin, and the faint blue shadow along his jaw said he had not been home. There was a paper cup beside him on the bench, empty, bent flat in the middle where he had crushed it. There was no Marco. There was no car at the door, she saw, when she looked up the stairwell. There was only a man on a bench.

A pale card hung on the wall above his head, a faded poster from some long-ago hospital campaign. Wash your hands. Save a life.

She thought: he has been here for two hours.

She knew it as certainly as she knew her own name. The guard would have told her, if she had asked. A man like Matteo Bellandi did not sit on a wooden bench in a basement for two hours for any reason that could be put in a ledger. He did it as other men pray: quietly, without display, because the alternative was to be somewhere else.

She made a small noise, and his head came up.

He got to his feet so quickly the coat slid off his knee. He caught it before it touched the floor, and stood holding it, with an expression she had no name for. Surprise. Relief. Then, tightly, a gentle and rueful embarrassment, as if he had been caught in a mildly foolish act and would have liked to deny it. He did not move toward her. He stayed exactly where he was, ten paces off, at the end of the long cold corridor.

"You said not to speak until you came out," he said. "I'm afraid I've kept to it too faithfully. I seem to have forgotten what to say."

"How long have you been here?"

"A little while."

"Matteo."

"Two hours," he said. "And ten minutes."

"You could have gone home."

"Yes." He looked at her steadily. "I'd rather have been here."

She did not know what to do with that. She came down the corridor with her cardigan clutched in one hand and her bag in the other, past the doors of the pathology storeroom and the mortuary and the long steel racks of the cold room, and her footsteps rang on the concrete in the silence. She was suddenly, absurdly conscious of her own appearance: the tangled braid, the stained cuffs, the memory of a headache behind her eyes.

The cold met her halfway. It was a damp, bone-deep basement cold, and she had not realized how it had worked its way through her thin cardigan until her teeth began, without her permission, to chatter.

Matteo saw it. His face changed.

He did not say anything. He crossed the last few paces of the corridor, not fast, with a slow, deliberate, almost formal tread, and stopped at arm's length. He shook out the coat he was holding, a long dark overcoat of heavy wool, lined with silk, still warm from his knee. He lifted it, and she thought for one wild second that he meant to put it around her himself.

He held it out instead, by the collar, open, like the door of a house.

"May I?" he said.

It was such a small question. It was a question one asked of a woman who had once, in another life, been dressed and undressed and pinned and laced by three women in a dressing room, without once being asked what she wanted. She felt it in the pit of her stomach.

"Yes," she said.

He stepped behind her. She stood very still, with her face toward the long dim corridor and her heart going like a bird, and felt the weight of the coat settle onto her shoulders, soft and heavy and impossibly warm. It smelled of cedar and starch and the faint salt breath of the sea. He drew the collar up around her neck with a careful, unhurried touch, and she felt the side of his thumb brush the nape of her neck, below her braid, light as a feather.

It lasted no more than a second. It went through her like a current.

She reached up to pull the lapels together, and her fingers closed over his, just for a moment, because he had not quite let go of the collar. Her hand was cold. His was warm. Neither of them moved. For a breath, two, three, they stood in the cold light with their hands joined on the lapel of a coat that belonged to him, and she could feel his pulse against her fingertips, quick and hard, as if he had been running. His other hand hung at his side, and she saw the knuckles whiten.

Then he drew his fingers out from under hers, gently, as though extracting a thing from water, and stepped back.

"Better?" Matteo said. His voice was lower than she had ever heard it.

"Yes," she said. "Thank you."

He nodded, and said nothing else, and the corridor waited around them.

They walked up the long ramp to the street in a silence that was not quite silence. Their footsteps made two separate sounds on the concrete, hers short and quick, his longer and slower, falling into a rhythm without either of them meaning it to. The pipes ticked over their heads. Somewhere a door opened and shut. He walked at her left, half a pace behind, with his hands clasped at his back, and she felt him there as she would have felt a lamp in a dark room. She did not look at him. She looked straight ahead, at the pale wedge of the exit door, and was aware, with painful precision, of the distance between her sleeve and his: four inches, three, four.

At the top of the ramp, where the cold air came in smelling of rain and diesel and wet leaves, he stopped.

"Sienna."

"Yes."

"I didn't come to ask what you found." He said it quietly, looking at the wet street, at the orange blur of the lamps. "I want you to know that. I came because it was late, and you were alone in a basement, and I couldn't have slept." A faint, wry breath. "I haven't been sleeping, as a rule. It seemed a waste of an evening to sit in the study and not sleep in a more useful place."

She tightened her fingers on the coat. She felt the strangest pull in her chest, as of two currents running in opposite directions. The first was a long, cold, level voice that said: you are being managed. A man who knows how to wait on a bench in a basement for two hours knows exactly what a woman feels when he does it. He does not need to ask what you found. He knows already, or he does not need to know.

The second was a very small voice indeed. It said: he is cold. He gave you his coat, and he is standing in his shirtsleeves in a wet wind, and he has not once complained.

She did not tell him what she had found.

It was the single most deliberate thing she did that night. She stood in the doorway of the hospital with the sleeves of his coat hanging past her fingertips and the whole terrible elegance of it sitting behind her teeth: two harmless things that met in a glass, a bottle, a flute, a hand. She thought of clause seven. Neither party shall lie to the other about any matter concerning the other's safety. It was not a lie, she told herself, to be silent. It was a delay. It was a door held not quite shut.

But she knew, as she stood there, that it was also exactly what he had done to her. She had said Not before this year is a very careful sentence. And she was, at this moment, being very careful too.

She drew a long breath, and let it go.

"I need something from you," she said.

"Anything."

"Don't say that until you've heard it." She turned and looked up at him. In the streetlamp his face was all planes and shadow, and his eyes were very dark. "I want a list. Everyone who was in the house on the day of the wedding. Staff, guests, caterers, the man who delivered the ice. Who stood in the kitchen, who carried the bottles up, who handled the glasses, from the moment they left the cabinet to the moment they reached the table. And I want to know who cleaned the crystal Dario gave us, and when, and with what."

He was silent for a moment. A taxi hissed by, throwing a long wave of light across the wet street. A leaf spun down out of the dark and landed on his shoulder, and he did not brush it off.

She waited for the questions. Why. What have you found. What do you think happened. She had her answers ready, a careful scaffold of half-truths, built on the tram.

Matteo did not ask a single one.

"Marco will have it by morning," he said.

That was all.

She stared at him. She felt the old suspicion lift its head like a cat, and, beside it, with absurd force, something that was very much like gratitude. A man who held nothing back would have asked. A man who held a great deal back would also have asked, because it was the natural thing to do. Only a man who already knew the shape of the answer, or who trusted her entirely, would stand in the rain and say: by morning.

She did not know which of those men she was looking at. It frightened her that she could not tell.

"Thank you," she said at last.

"You needn't thank me."

"I know." She pulled his coat closer around her shoulders. "I'm going to anyway."

The corner of his mouth moved. It was not a smile, quite. It was only the memory of one, a thing he had put away a very long time ago and had just remembered where he had left it.

The black car was waiting at the end of the street, with its lights off and the engine silent. She had not seen it from the door. Marco stood beside it in his dark coat, hat pulled low, looking up at the sky as if he had been appointed to watch the weather, and as she and Matteo came down the steps he opened the rear door without a word.

She slid in. The seat was warm. The coat smelled of him, and she did not take it off.

In her notebook, that night, in the long green quiet of the east suite, with the bolt shot home and the lamp turned low, Sienna wrote four lines in the Divergence Log. She wrote them slowly. She wrote them very carefully, the way one sets down a thing one is afraid to look at.

Last time, the poison was never in the glass. It was in the meeting.

Last time, no one knew who cleaned the crystal. This time, I asked.

Last time, no one sat outside my door.

And below it, in a hand so small that it almost vanished into the margin: He did not ask me why.