LONDON — The most eagerly awaited play in modern theatrical history has begun with a bang, a whizz, and plenty of shocking plot twists.

Just a few hundred people now know what happened to Harry, Hermione, and Ron in the decades after leaving Hogwarts.

All three characters and their children star in the eighth installment of the Harry Potter series, which J.K. Rowling has chosen to present as a live theatrical production in London.

Harry’s denouement, Harry Potter and the Cursed Child, is so epic that it will be performed in two parts. Part I played to a paying audience for the first time on Tuesday and induced squeals, gasps, and laughter, according to the Harry Potter super fans who secured tickets. They will have to wait until Thursday to see the second 2 1/2-hour show.

The play starts with the same scene that ends the books and films—Harry Potter is on Platform 9 3/4 at Kings Cross station in London, waving his son Albus off to Hogwarts for the first time.

Rowling herself, who has asked the audience to keep the plot secret, hinted that Scorpius, the son of Draco Malfoy, would also play a prominent role.

“I was totally surprised by the story,” said Anna Sampson, 28, one of the lucky people who saw the first night. “The special effects really do make it magical.”

The actors are hurled about on wires, sometimes vanishing into a puff of smoke and transforming into one another. A real owl even swoops over the audience.

The only real first-night hiccup saw the owl escape from its handler and spend the end of the first half flapping around the auditorium.

Samantha Kent, who was dressed in a Hogwarts school uniform, had come down from Birmingham to see the show. “The choreography and stagecraft was stunning,” she said.

The 26-year-old Harry Potter fan-fiction writer said she had spent years imagining what might have happened to Harry and his friends. “I kind of had an idea how it would go,” she said. “I was completely wrong!”

One plot shock gives rise to an entirely new Harry Potter symbol. “I’m going to have to get a new tattoo,” said Kent, motioning toward the deathly hallows, snitch, and dark mark already inked into her left forearm.

The play was written by Jack Thorne and it’s described as being “based on a story by J.K. Rowling, Jack Thorne, and John Tiffany.” Rowling may not have written the script herself, but she was closely involved throughout.

“I would say it’s assisted and approved fan fiction,” said Kent, an expert in the genre.

The story will be published as a book at the end of July, to coincide with the show’s official first night after eight weeks of previews. Harry Potter fans unable to get their hands on the most sought-after tickets in West End history will have to wait.

Before the show opened, there was a furor about the casting of an award-winning black actress to play Hermione Granger, but Rowling has vigorously defended that decision, and the audience agreed unanimously that Noma Dumezweni was spectacular.

Chris Bewsher, 18, and his mother, Amanda, from North Wales, said the new play was better than the Hollywood interpretation. “It now goes books, play, movies, in that order,” said the student.

Stuart Williams, 27, said he was surprised by how often he had laughed. “The films got progressively darker, but there was always humor in the books. It was a lot funnier than I expected,” he said.

It was also more shocking—with a genuinely gasp-inducing surprise at the end of Part I. “That was a good cliffhanger,” Williams said.

One theatergoer, who asked to remain anonymous, warned Harry Potter fans that the twist may force them to “imagine the impossible.”