Mr. Serpico still carries the detective shield he was awarded as he left the department on a disability pension and, often, his licensed revolver, with which he takes target practice on his 50-acre property not far from this Columbia County hamlet. He also still carries bullet fragments lodged just below his brain from the drug shooting; he is deaf in his left ear, and has nerve damage in his left leg.

For many, “Serpico” conjures the face of Al Pacino, who won his first Golden Globe award for his star turn in the film. The movie  along with news reports and the best-selling biography of the same name  seared the public memory with painful images: of the honest cop bleeding in a squad car rushing to the hospital, where, over months of rehabilitation, he received cards telling him to rot in hell. Instead, Mr. Serpico took his fluffy sheepdog, Alfie, and boarded a ship to Europe; the film’s closing credits describe him as “now living somewhere in Switzerland.”

Which was true at the time. After years traveling abroad, Mr. Serpico returned to the United States around 1980 and lived as a nomad, out of a camper. He finally settled about two hours north of New York City, where he lives a monastic life in a one-room cabin he built in the woods near the Hudson River. In 1997, he spoke out after the brutal beatings of Abner Louima in a Brooklyn station house, but mostly he stays far from his old nemesis.

Now, all these years later, Mr. Serpico is working on his own version of the harrowing adventures chronicled by Peter Maas’s biography, which sold more than three million copies (royalties from the book and the movie have helped him live comfortably without working). The memoir begins with the same awful scene as the film: Serpico shot in the face during a heroin bust on Driggs Avenue in Williamsburg, Brooklyn, Feb. 3, 1971. Working title: “Before I Go.”

“It’s the rest of the story,” he said recently over lunch in the self-service cafe of a health-food store here in Harlemville. “It’s more personal. I used to think, ‘How can I write my life story? I’m still living it.’ ” Though he is healthy, he added, “I’m getting close to the line, so I figure I better get busy.”