By CHARLES McGRATH

On a blistering afternoon last June, outside a Polish social club, men in heavy wool tuxedos, with slicked-back hair and pencil-thin mustaches, were blotting their brows. Nearby were some very slender young women in spangly, ankle-length dresses. But because this was Brooklyn, where people wear weird clothes all the time, nobody paid them attention.

A few blocks away, a seaside boardwalk had miraculously arisen, complete with clubs, restaurants and snack vendors. Except that the shops were empty. And where the ocean should have been, there was, instead, a wall of metal shipping containers. This brand-new ghost town is the $5 million set for “Boardwalk Empire,” an HBO series that begins September 19.

“Boardwalk Empire” is set in Atlantic City, New Jersey, in 1920, during the first year of Prohibition, and the big outdoor set, the vintage clothing and historical research are all evidence of the unusual, painstaking lengths the show’s creators have gone to recreate a mostly unfamiliar era. Martin Scorsese directed the pilot episode and became an executive producer of the series.

Prohibition, which made the sale and manufacture of alcohol illegal, was tied both to the introduction of the income tax and universal suffrage, and radically altered the relation of citizen and government. “Prohibition is like a guilty secret, or an embarrassment,” said Daniel Okrent, a former public editor for The New York Times, who has just published a history of the period, “Last Call: The Rise and Fall of Prohibition.”

“How do you explain that for 13 years there was an amendment to the Constitution of the United States that said you couldn’t get a drink legally? It beggars the imagination.” “Boardwalk Empire” is based in part on a book by the same name . In 2006 HBO showed the book to Terence Winter, who wrote many “Sopranos” episodes for the network. “I’ve always loved the way people talked in the ‘20s, and the clothes, the cars,” he said.

“It was such a transitional period. The world was changing so much. And in some ways it was a very modern time. This was almost a hundred years ago, but they had airplanes, telephones, people went to the movies all the time.” Mr. Winter focused on the ‘20s and Enoch Johnson, known as Nucky, by far the most vivid character in the book.

Nucky was a political boss and stalwart of the Republican Party who from 1911 to 1941 controlled all the vice in Atlantic City. He occupied a whole floor of the Ritz-Carlton hotel, rising every day at 3 p.m. to travel in a powder- blue Rolls-Royce. The real Nucky was tall and broad-shouldered, with an enormous, domelike head.

In the show, fictionalized slightly as Nucky Thompson, he’s played by the bugeyed, slightly cadaverous Steve Buscemi, another “Sopranos” alumnus. “ My inspiration for Nucky was the Prince of Wales,” said John A. Dunn, the show’s costume designer, referring to the dandy who later became Edward VIII.

Mr. Dunn was standing recently in a storage room in a Brooklyn soundstage . Nucky’s suits were on a rack next to Al Capone’s and near Arnold Rothstein’s and Lucky Luciano’s. (They are also characters in “Boardwalk Empire.”) Mr. Dunn used vintage clothing, either rented or bought on eBay or in vintage clothing shops; otherwise the costumes were handmade.

“The great surprise for me was the color,” he said. “Because of photographs we tend to think of ‘20s clothing as black and white, but really there was this splash of new, bold color, maybe in reaction to World War I.” The show’s music, bright and ebullient, is also authentic and also a reaction to the end of the war. People wanted to get up and dance, as Mr. Okrent pointed out, and Prohibition, or the speakeasy culture, conveniently (and for the first time) mingled men, women and alcohol in an atmosphere of congenial illicitness.

Some of the show’s tunes haven’t been heard for close to a century. “Marty and Terry both wanted the music to be historically accurate,” said Randall Poster, the music coordinator for the series. “So we just immersed ourselves in this fascinating transitional period when ragtime is just beginning to turn into jazz. ” The research even extended to the way people talked and what they read in the ‘20s.

One of the characters is reading a novel by Henry James; another keeps a copy of Sinclair Lewis with him. “I hate to say it, but before TV people spoke better and were better read than we are,”

Mr. Winter said. “They were probably more literate.” And they also had their vices. “We have whiskey, wine, women, song and slot machines,” the real Nucky once said. “I won’t deny it, and I won’t apologize for it. If the majority of the people didn’t want them, they wouldn’t be profitable.”