George Washington loved a pinny. He wore one to lay the cornerstone of the Capitol building in 1793 and it wasn’t just to stop his trousers getting grubby. If you look at one of his statues, in the US National Cathedral in DC, words carved into the base describe the man thus: patriot, soldier, first citizen, president, churchman, statesman, farmer, freemason.

The pinny was a mason’s apron and the influence that this secretive trade organisation had on the leader and the city is one of Washington’s most intriguing facts.

It was a lesser-known fact. But Dan Brown’s new book, The Lost Symbol, which sold more than a million copies on its first day of publication this month, is set entirely in Washington DC and has Freemasonry at the heart of its plot.

People are going to be following Brown’s hero Robert Langdon in a story that not only reveals an alternative to Washington’s tourist sights, but veers alluringly off the beaten track.

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Hence I found myself 13 blocks north of the White House outside the House of the Temple, a Masonic headquarters that features in the sinister opening scene of Brown’s book. The architecture mimics an ancient wonder of the world, the Mausoleum of Halicarnassus, with soaring columns, a pyramid and gargantuan doors guarded by two sphinxes.

The edifice gave me the shivers. I walked shyly in, clutching my freshly thumbed copy of The Lost Symbol. A young chap called Jason eagerly stepped from behind a desk — where lay a copy of the same book. He cheerily explained that he was mugging up.

I expected a limited tour, but Jason led me straight to the temple itself. It was staggering. We were standing under a vast dome in an ornate chamber where ceremonial hand-carved walnut seats and a huge throne for the “Grand Master” surrounded a large, central, jet-black marble altar.

Jason didn’t hesitate. He knelt at the altar and said: “This is where that opening scene takes place in the book, where the villain drinks the red wine out of the skull.” He held up cupped hands as if to demonstrate the secret ritual.

He then led the way through the library, where Brown researched the novel. We passed portraits of out-and-proud Masons, such as President Washington, both Roosevelts, Harry Truman and Gerald Ford; also J. Edgar Hoover, the astronaut John Glenn and the golfer Arnold Palmer. Masonry is much less clandestine in the US. But no less unsettling, I decided.

Did Jason believe that Washington DC was designed so that landmarks and avenues formed masonic symbols? He said no. Then maybe. Langdon pooh-poohs the notion in the book, though it’s a favourite of conspiracy theorists and will undoubtedly feature in the Lost Symbol tours that are expected to materialise within weeks.

I walked up to the gleaming Capitol, where gruesome action takes place in the novel. In real life it’s buzzing with crowds, who pause to marvel at the painting on the ceiling, The Apotheosis of George Washington. The tour also visits the crypt — Brown loves a crypt — where a marble compass in the floor marks the dead centre of the Capitol and the point from which the streets all radiate.

Brown also loves a tunnel, so it’s exciting to use the public tunnel from the visitors’ centre to the Library of Congress. The Great Hall is so beautiful I got a lump in my throat. Grand marble columns and statues stretch up to ceiling murals of cherubs, stars and sea creatures, while a balcony overlooks the spectacular octagonal reading room. Langdon’s journey through these places is terrifying, while mine was merely exhilarating.

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