An e-mail explained that improper attire could get you bounced from the Passing Fancies party before you even stepped foot in the town house. Even if you were a member. Even if you had already bought a ticket. “Improper” didn’t mean indecent; several girls wore just lace bras and thongs. It meant jeans, uncombed hair. “Improper” meant looking bad.

Passing Fancies, a London sex club for people under 30, was hosting a party in an area of the city a friend described as “full of embassies.” The club is part of the “Eton-verse,” that friend said. It’s for boys who went to the British boarding school started by Henry VI or one of its select equivalents, and for girls who choose to associate with the only boys in England with a chance at becoming prime minister. Men wore suits, and most women wore cocktail dresses.

People can apply for membership online, via a link you can only get from an existing member. The first question is: Whom do you know in the club? No one knows who reviews applications.

This party was a get-to-know-members event. The one three weeks later would be an orgy.

It’s for boys who went to the British boarding school started by Henry VI, and for girls who choose to associate with the only boys in England with a chance at becoming prime minister.

“A lot of my colleagues go to these parties,” said a 20-year-old girl with bleached hair and toothpick thighs. I asked where she worked. “I’m a high-fashion model.”

The venue, an opulent four-story town house, had almost no furniture, just a few couches. In one room, a group of boys parked a small tank of nitrous oxide meant for blowing up balloons. They swallowed the gas in big gulps. A boy by the tank who wore a salmon-pink pocket-square pointed at a boy across the room and whispered to me, “I recently went on a date with him. I don’t want to see him tonight.” Then he recommended five restaurants in Copenhagen that, according to him, had excellent tasting menus.

The only art on the walls were John Constable–style landscape paintings with many spaniels and no people in them.

A ginger girl in a black lace bra—nipples visible—said she came to the event with a date. She met him on an app for people looking to have kinky sex. As she said this, I totally forgot she was wearing only underwear. She seemed to forget, too.

In Los Angeles, I’d assume everyone at a party like this had parents in the entertainment industry. New York City, parents in finance. What about London? Without pause, two boys answered, “Politics.”

The only art on the walls were John Constable–style landscape paintings with many spaniels and no people in them.

Around midnight, people migrated like birds to the first floor, forming a circle in a sprawling room with a white marble fireplace. A woman in lingerie walked into the center. From her garter, she pulled out a 12-inch-long balloon that looked like a hot dog. She danced while licking it. Her breasts—maybe fake, maybe real, definitely massive—bounced up but never really down. Then she tilted her head back and slowly pushed the balloon down her throat until it disappeared.

“Oh … my … God …,” said the guy in front of me, who wore a coat and tails.

The balloon girl pulled another girl wearing lingerie into the center of the circle. Maybe they knew each other, maybe they didn’t. They simulated having sex but kept their clothes on. People clapped politely.

The third floor was roped off by a security guard wearing all black. “V.I.P. only,” he said, not moving. A girl wearing the largest choker Vivienne Westwood ever made was let upstairs.

In Los Angeles, I’d assume everyone at a party like this had parents in the entertainment industry. New York City, parents in finance. What about London? Without pause, two boys answered, “Politics.”

The bar was in the basement, along with a D.J. who, miraculously, couldn’t be heard on the higher floors. Down there, an intense Spanish man in his late 20s eyed the ginger girl in lingerie. He walked to her, gripped her neck, and held her up against the wall. Six inches away from me. I looked over at the ginger girl, who was smiling real big. I left to get a gin-and-tonic.

In the D.J. room, thick red curtains gave small side rooms the option of privacy. People danced slow side-to-side shuffles. Across the dance floor, I saw the Spanish guy. He was in a side room getting fellated by the ginger girl. They decided not to close the curtains.

Around one A.M., about 50 people again migrated to the white-marble-fireplace room. People formed a circle around an oak table in a corner of the room. A girl wearing red pasties and a G-string crawled onto it and lit a rod on fire. She held it near her crotch, then shoved it down her throat. No one said a word.

Around three A.M., a lanky boy asked if I was enjoying myself. He was the host, I guessed. “Yes, but I’m tired,” I said. He pointed toward the grand, spiral staircase, where a man in a suit sat. “Go tell him you’re tired and he’ll give you just the thing. That’s exactly why he’s here.” The party had four more hours to go.

Bo Derrière is a London-based writer. She is the author of several books, including Fifty Shades of Gris: A Frenchwoman’s Guide to Getting Down but Staying Posh in London; Pulp Diction: The Old-School Guide to Dirty Talk; Heimlich Hiatus: A History of Choking and Sex; and Moscow Mule: The Drug Runners of Soviet Russia