It’s not every day that a Brooklyn allergy doctor is alerted by his receptionist that a stranger is standing in his waiting room claiming that a Cuban-born spy named Sanchez once lived in the building. But it happened to Dr. Norman Horace Greeley a few weeks ago in his home office at 140 Clinton Street in Brooklyn Heights. “I thought he might’ve been a nut case,” Dr. Greeley recalled, “and basically I turned him away.”

But the visitor, the historian John Harris, tried one last bid to get a tour of the 1850s townhouse: He left the doctor a copy of his book, “The Last Slave Ships,” along with a hastily scrawled note intimating that a famous ancestor of the doctor was linked to Emilio Sanchez y Dolz, the 19th-century spy whose biography Mr. Harris is writing for Yale University Press.

Though Mr. Harris had arrived on the building’s doorstep knowing nothing of its current occupant, he had immediately been intrigued by Dr. Greeley’s name and by a portrait in the waiting room of Horace Greeley, who was the renowned founding editor of The New-York Tribune and an 1872 presidential candidate.

“It was mysterious and tantalizing,” Mr. Harris said of the portrait. “I’m working with an obscure figure” — Mr. Sanchez — “who deserves to be as famous as Greeley, and here he is connected to” Greeley himself, “a much greater figure from the period.”

Five minutes later, as Mr. Harris sat on the steps of a church across the street, Dr. Greeley called him, bursting with curiosity. He was indeed a descendant of the Tribune editor, he told Mr. Harris, and the house had been continuously occupied by Greeleys since the early 1900s. What’s more, one of the doctor’s sons, Matthew Greeley, was shooting a short-form documentary about the famously antislavery family patriarch.