The week before Labor Day, 2016, Circe Hamilton, a freelance photographer in her mid-forties, was preparing to move back to the U.K., after twenty years in New York. She had begun to think of the city as an obstruction; she had recently struggled to make a living, and felt that she was depriving her young son of a gentler, slower childhood in west London, with access to English relatives, the National Health Service, and muddy playgrounds under gray skies. Hamilton is an American citizen—and, she recently learned, a descendant of Alexander Hamilton—but she grew up in England, sounds English, and has a British passport. When her friend Valentina Rice hosted a farewell dinner for her, on August 30th, Hamilton was surrounded by expatriate British women with careers in the media and in fashion. The guests ate blueberry polenta cake and said goodbye to someone they understood to be a single mother.

Hamilton’s son, Abush, was born in Ethiopia, and was adopted by Circe in August, 2011, when he was a toddler. A year and a half earlier, Hamilton had broken up with Kelly Gunn, the woman who had been her romantic partner for several years. In their final year together, Hamilton and Gunn had begun the process of an overseas adoption. After the separation, Hamilton continued to pursue the process. The two women remained in close contact, and a year after Abush arrived Gunn became his godmother. Despite some friction between the women about the meaning of that role, Gunn and Abush developed a strong bond. He often stayed with her overnight; he loved her dogs.

On the morning of the farewell dinner, Hamilton had walked with Abush from her home, in the West Village, to Gunn’s apartment, on Sullivan Street, a block south of Washington Square Park. The apartment is modern, with glossy dark floors and a wall of windows. Gunn had become wealthy by supplying Apple with display fixtures for its stores; she had run her own design company, and had been a partner in another. She owned property in Los Angeles, and a summer house on Fire Island. She had offered to take Abush to the beach for a few nights while Hamilton finished packing. Hamilton would join them on Thursday, September 1st, and then bring Abush back to New York before flying with him to London, on Saturday night.

Hamilton is tall, with long hair and a long, pale face. Gunn, who is fifty-three, has cropped graying hair and wears round white-rimmed glasses on a round face; she prefers adventurously billowing clothes made of dark fabrics. Compared with Hamilton, who is unimpressed by displays of emotion, Gunn is happier to use the language of therapy, and is readier to share her feelings. Disorder can agitate her—she once sent an employee to her home to deal with an insect—and on the day of Hamilton’s farewell party Gunn was upset about a blocked toilet. She had mentioned it in a text message, but was repelled when Hamilton carried her own plunger across town and into Gunn’s apartment. Hamilton later recalled that, after she dropped off Abush with Gunn, she thought that her ex seemed “more panicked than usual.”

The next day, Wednesday, a shipping company collected Hamilton’s belongings. She had what she thought would be her final photo shoot in New York: a portrait of Emma Forbes, a British TV presenter, for Hello! Gunn later sent her pictures of Abush having fun at the beach.

At one o’clock on Thursday, Hamilton was at home cleaning, expecting to leave for Fire Island in the evening, when she got a call from a woman who introduced herself as Nancy Chemtob. A New York family and matrimonial lawyer, Chemtob founded her own firm in her twenties; in the two and a half decades since, she has represented such clients as Bobby Flay, Star Jones, and Diandra Douglas, the ex-wife of Michael Douglas, in divorce proceedings. Her style is amused and unsentimental, and she has a strong Long Island accent. (Today, when Hamilton and Chemtob refer to each other, they use inexpert, mocking approximations of the other’s accent.)

Chemtob told Hamilton that she represented Kelly Gunn. Hamilton only half-registered what came next. Chemtob recalls telling Hamilton that Gunn had just asked a New York court to recognize her as one of Abush’s parents and award her joint legal and physical custody. As an interim measure, Gunn was seeking a restraining order that would stop Hamilton from taking him out of the country. Chemtob told Hamilton that, at 2:30 p.m., she must appear before a matrimonial judge on Centre Street. She should bring Abush’s American and British passports.

Hamilton began to shake. “I fell apart,” she said recently. (Chemtob, recalling Hamilton’s shock, said, “She had no clue.”) Hamilton changed, got in a taxi, and called Valentina Rice. Rice began asking friends to recommend lawyers, and one of them spoke to a family-law specialist, who said that Hamilton should “get the hell out of there.” Without legal representation, she was “walking into an ambush.” Rice relayed this advice, but Hamilton, she told me, “felt she had to go, and she didn’t have her son.”

In the courtroom, Gunn and Hamilton didn’t speak to each other. “It was an out-of-body experience,” Hamilton recalled. “I thought I was in a really weird play: ‘Where am I, and how did I end up here?’ ”

Chemtob told the judge that Gunn was in a “co-parenting relationship where the child one hundred per cent believes, and knows, that he has two mothers.” Gunn and Hamilton had raised Abush “as both parents equally.” She acknowledged that Abush usually called Kelly Gunn by her first name—truncated to “Kee”—but only because Gunn and Hamilton had agreed that “ ‘Ma’ and ‘Mommy’ would be confusing.” Hamilton was a “flight risk,” Chemtob said, and Gunn had become “very concerned about the welfare of the child.”

The judge invited Hamilton to speak. “I have no idea why I was brought into the courtroom,” she said. “I am the sole parent.”

The judge allowed Gunn’s petition to progress, and Hamilton relinquished Abush’s passports. Leaving the courtroom, she briefly embraced Gunn, who was weeping, and whispered, “I’m so sorry.” Hamilton later told me, “I did feel sorry for her. It was ‘Why do you have to do this?’ ” Hamilton then cancelled her flight, reënrolled Abush in school, and hired a lawyer.

Gunn v. Hamilton—an inquiry into whether Abush had two parents or one—began the following week, and was still running in the new year. The proceedings, which exhumed hundreds of e-mails of love and regret, became an intimate history of a New York romance and its aftermath: a study of what counts as splitting up, what counts as a family, and, in a quiet but stubborn subtext, whether the ability to pay for good dentistry enhances a legal claim to be something more than a godmother.

The case was the first of its kind in the city. “It’s as if you gave me the keys to your apartment and, suddenly, I’m saying, ‘The apartment is mine,’ ” Hamilton told me, bleakly, last fall. “What the fuck? Where does it end?” Her life had been put on hold, and her possessions were stuck in a shipping warehouse in New Jersey. Abush had returned to school; Hamilton couldn’t take him out of state without permission. The court had allotted Gunn time with Abush on Sundays and on Thursday afternoons.

Several times a week for months, Hamilton and Gunn sat a few feet from each other in a bright, shabby courtroom, at 80 Centre Street. A sign on the wall noted that “loud and angry words generally indicate a weak argument,” but the white noise of the city, through open windows, risked drowning out any form of speech gentler than a reprimand. Abush was not in the courtroom, but visitors in the public seats sometimes glimpsed his image when attorneys looked at e-mail printouts with photo attachments: a smiling boy with big eyes and a high forehead, playing with a dog or being held in the air.