In chemistry, a catalyst is a substance that accelerates a reaction without being consumed by it. This is the small, ironic truth about catalysts: they make things happen and then they are still there, unchanged, while everything around them has been transformed. There is something almost unfair about this, and something almost beautiful, and a woman named Maeve Sullivan who lived through the summer of 1925 came to understand both.

Chicago in 1925 was a city running on two fuels: ambition and bootleg whiskey. The ambition came from the people who had arrived from Europe and from the South and from places whose names were pronounced incorrectly by the natives, carrying with them a hunger that was itself a kind of energy, a catalytic force that was turning factories into empires and streets into corridors of light. The bootleg whiskey came from the Canadian border and the Great Lakes and a network of tunnels that connected basements on opposite sides of every major street, and it was the visible manifestation of something larger: a society that had declared a substance illegal and had then spent the next five years becoming addicted to it, creating not prohibition but opportunity, not order but a new kind of chaos that wore a suit and carried a gold watch and called himself a businessman.

Maeve was twenty-four and she worked for the Chicago Department of Charities and Corrections, which was the official name for what everyone called the poor office, a place where women who had lost husbands or jobs or children went to ask for money that was never quite enough and forms that were always incomplete and answers that were always the same: there is no more aid available for this category, but you may come back in six weeks and we will review your case. Maeve had been born in a small house on the West Side to parents who had come from County Cork, and she had grown up understanding that the city had two temperature zones, as the earlier generation had called them, or two speeds, or two versions of reality: the city that was built and the city that was built upon.

She was assigned to a new program in the spring of 1925, a pilot initiative called the Neighborhood Wellness Project, which was a bureaucratic name for something radical: she was to visit the same families every week for a year and get to know them, really know them, not as cases but as people, and report back on what she found, and the idea—naive, perhaps, but not entirely wrong—was that you cannot solve a problem that you do not understand, and you cannot understand a problem that you see only in a snapshot, because problems, like people, change from week to week.

The family she was first assigned to live on South Halsted Street in a building that had been a factory forty years ago and was now divided into thirty-two apartments that shared three toilets and a kitchen sink and a staircase that groaned under the weight of so many generations living on top of each other. The mother, a widow named Rosemary Delaney, worked as a seamstress for a garment manufacturer who paid by the piece and whose prices dropped every year while the rent rose, and she had two children: a boy of nine named Tommy who had stopped going to school because he was needed at home, and a girl of six named Kathleen who was sick.

Kathleen was sick with pneumonia, which in 1925 was a disease you either survived or died from, with no intermediate state, and Maeve visited three times a week and brought what she could—broth from a kitchen she did not own, blankets from a cupboard she did not manage, and the particular form of attention that a trained social worker could offer, which was the ability to sit in a hot room and listen to a woman cry and not try to stop her and not try to fix her, just to be there while she was crying.

And it was on one of these visits, in the third week of May, that Maeve met Jack O'Malley.

He came to the door asking for Rosemary and she told him that Rosemary was in the back room with the children and he said thank you and she said you are welcome and he stood there in the doorway wearing a suit that was expensive but not new, which meant he could afford the fabric but not the tailor, and he had a face that was all angles and energy, the kind of face that looks like it has never wasted a moment on anything unnecessary, and he said, I am Jack O'Malley, and she said, I know who you are, which was true: Jack O'Malley was one of the younger bootleggers in Chicago, a man who had built a small empire on the South Side and who was known for two things: his violence, when he chose to be violent, and his discretion, when he chose to be discreet. He chose discretion most of the time.

I am not here to cause trouble, he said, as though she had accused him of intending to. Rosemary used to work for my mother, before my mother died. She sews the shirts I wear to church. I just wanted to check on her.

Maeve looked at him, and something in her professional training told her to be careful, and something else told her that caution was not the same as wisdom, and she said, Come in.

What happened next was a small thing, almost nothing: Jack O'Malley came into the back room, where Rosemary was feeding Kathleen broth, and he stood at the foot of the bed and he looked at the sick child with an expression that Maeve would remember for the rest of her life, which was the expression of a man who has spent his life making things worse and has suddenly, unexpectedly, wished to make something better, and he said, This is my sister Kathleen. And Rosemary looked at him sharply and said, She is not your sister. And he said, I know. But she could be. And Rosemary, who had been widowed for three years and who had been carrying the weight of two children alone and who had not allowed herself to cry in front of anyone since the funeral, sat down on the edge of the bed and began to cry, and Jack O'Malley sat down next to her, and Maeve stood in the doorway and realized that she was witnessing something that she could not categorize, because it did not fit into any of the frameworks she had been taught.

She had been taught that people were either victims or perpetrators, or sometimes both, but that the categories were useful, and that understanding who was who was the first step to helping them. But Jack O'Malley did not fit. He was a bootlegger, which made him a perpetrator in the official story, and yet he was sitting next to a widow who was a victim of the same system that he had learned to navigate, and he was looking at her sick child with a tenderness that was completely unprofessional and entirely genuine, and Maeve understood, in a moment that she would describe later as catalytic, that the reaction between these two people was not going to be simple. Catalysts do not determine the direction of a reaction. They only accelerate it. And the reaction between Jack O'Malley and Rosemary Delaney was already underway, powered by forces that Maeve could not control and did not understand.

She should have stepped back. A cautious person would have stepped back. But Maeve was not cautious. She was twenty-four and she had been trained to intervene, and so she intervened, in the only way she knew how: she began to visit more often, and she brought not just broth and blankets but information—information about health services, about legal aid, about a doctor on South State Street who treated patients on a sliding scale, about a food pantry that delivered to homes, about a program that might help Tommy return to school. She brought this information to the Delaney household and she brought it to Jack O'Malley, who had begun appearing at the door unannounced, usually in the evenings, usually with something in his pocket that he offered to Kathleen when she was well enough to receive visitors—a candy bar that Maeve knew was probably bootleg candy, manufactured in an kitchen that the health department had never inspected, but that Kathleen accepted with a seriousness that suggested she understood the weight of the gift.

And what Maeve began to observe over the following weeks was a transformation that was both small and enormous, a series of micro-changes that accumulated into something that she could not have predicted and could not have engineered. Jack O'Malley began to use his resources for things other than the bootleg trade. He knew a doctor who could see Kathleen for free. He knew a man who could fix the heating stove in the Delaney apartment. He knew a teacher who could come to Tommy's house three afternoons a week and help him with his lessons. He did these things in a way that was careful and discreet, which meant that Rosemary never felt obligated and Tommy never felt ashamed and Kathleen never knew that any of it was happening, because Jack was clever enough to route the help through Maeve, so that it always appeared as though the charity was coming from the department, from the doctor, from the teacher, never from him.

And Rosemary, for her part, began to do things that Maeve did not expect. She began to cook for Jack, which was not unusual—she was a good cook and he was hungry, and the arrangement was mutual and simple—but she began to cook things that required time, and time was the one thing a seamstress did not have, and so she began to work later at night, and her hands began to shake, and Maeve noticed and said nothing, because saying nothing was sometimes the most respectful thing you could do.

The catalyst accelerated the reaction. This is what catalysts do. They do not change the destination. They only make the journey faster. And the journey between Jack O'Malley and Rosemary Delaney was faster than anyone had expected, because it was powered not by romance or infatuation but by something deeper and more dangerous: recognition. They recognized in each other the same thing that Maeve recognized when she looked at them: a fundamental honesty, a refusal to pretend that the world was fair when it was not, a willingness to do what needed to be done without announcing it or seeking credit or performing it for an audience.

The trouble began in June, when Jack's operations attracted the attention of a larger gang, a Chicago organization led by a man named Frankie Moretti, who did not share Jack's discretion and who did not appreciate Jack's refusal to pay protection money that was, in Moretti's understanding, both customary and non-negotiable. The conflict between them was not something Maeve understood, and it was not something she should have understood, and yet she understood it anyway, because the world of the South Side was not separate from the world of the poor office and the sick children and the seamstresses who worked at night, and nothing in a city was ever separate from anything else.

On a Wednesday in mid-June, three men came to the Delaney apartment at midnight and they asked Rosemary a question: where is Jack O'Malley? And Rosemary, who was a brave woman in the way that poor women are brave because they have no other option, said, I don't know. And the men believed her, or pretended to believe her, and left, and Maeve found out about it the next morning when Rosemary told her, quietly, without fear, because she had already passed through the stage of fear and was living now in the stage that some people call despair and others call clarity.

Maeve did what she could. She called her contacts at the police department, which meant calling a man named O'Brien, who had been helpful to her in the past and who told her, in a voice that was not unkind but was entirely practical, that this is not our problem, Miss Sullivan, this is the gang problem, and we do not intervene in gang matters unless someone is killed, and it is too late to prevent what is coming and too early to investigate what has happened. So you are saying there is nothing you can do. I am saying that my hands are tied, Miss Sullivan. And she understood what his words meant: that the system had categories for different kinds of violence, and that gang violence, which was committed by men who could afford lawyers and bribes and political connections, occupied a category that was separate from and higher than the violence committed by women who could not, and that her authority, as a female social worker for the city, extended only to the lower category.

She went to see Jack that evening. He was in a basement apartment on South Plymouth Court, a man who was small but who occupied space like a man who was large, and he was sitting at a table with two other men and a bottle of whiskey, and he looked up when she entered and his face changed in a way that Maeve would remember, because it was the face of a man who had spent his life wearing armor and had suddenly, unexpectedly, found himself without it, and he said, Maeve, and his voice was gentle, and he stood up and he said, you should not be here. And she said, I know. And he said, I am sorry. And she said, for what? And he said, for everything.

She left and she walked home through streets that were loud and alive and dangerous in the way that streets are dangerous when they are full of people who have too much energy and not enough to do with it, and she thought about catalysts and about how they work, and she thought about how Jack O'Malley had entered her professional life as a variable she had not anticipated, a third element in an equation that she had thought was simple, and how he had accelerated a reaction between himself and Rosemary Delaney that she had not seen coming, and how she had been powerless to prevent it, not because she was weak but because the system she worked for was designed to manage symptoms, not causes, and Jack O'Malley was not a symptom. He was a cause. He was a cause that was accelerating toward a result that the city was not prepared to understand.

Moretti's men came for Jack on the third of July. They found him outside a warehouse on South Halsted where he was storing a shipment of whiskey that he did not intend to sell to Moretti, because he had decided, in the two weeks since Maeve had visited him, to get out of the business, to use the money he had made to do the things he had been doing for the Delaneys on a larger scale, to build a community center, to hire teachers, to pay for clinics, to invest in the kind of thing that a catalyst does: accelerates the good reactions and hopes that they outpace the bad ones.

They found him and they shot him three times in the chest and they left him in the street and the street, which was full of people who had spent their lives learning not to look at violence, did not look, and the body lay there until morning, when a man named Santos, who delivered milk to the warehouse and who was a good man in the way that poor men are good because goodness is the only luxury they can afford, found him and covered him with a milk crate and went to tell Maeve.

Maeve went to the warehouse and she saw Jack O'Malley lying in the street and she understood, in a moment that was not dramatic or cinematic but quiet and devastating and utterly real, that a catalyst can be consumed by the reaction it accelerates, even though the textbooks say they are not. Jack O'Malley was gone, and the reaction continued: Rosemary Delaney, who buried him with a ceremony that was small and private and entirely proper, went back to sewing and raised her children and lived to be seventy-two and told them, on her deathbed, about the man who had sat at the foot of her bed and looked at her sick daughter and called her his sister, and the children told their children, and the story continued, accelerated, transformed, carrying the energy of a man who had spent his life in the wrong profession but who had, in his final weeks, found the right one: he had become, briefly and imperfectly and humanly, a man who tried to make things better.

Maeve Sullivan retired from the Chicago Department of Charities in 1931, when the Depression made her job obsolete, because there was no more charity to distribute—everyone was poor, and the department became a different kind of department, one that handed out bread and beans and hope in measured quantities. She lived in a small apartment on the West Side and she never married and she never forgot the summer of 1925, and she kept, in a drawer of her desk, a photograph of a man she had known for six weeks, and when people asked her who he was, she said, He was a catalyst. And they did not understand, and she did not blame them.