The world of Harry Potter has become an industry. The books have sold hundreds of millions of copies around the world, the film adaptations raked in billions of dollars, and there are now real-life Hogwarts castles at Universal theme parks. But the latest creation born from the fantasy franchise is a decidedly old-school entity: it’s a play, Harry Potter and the Cursed Child, showing exclusively in London.

Most people won’t ever get to see the staged version for themselves—it’s already sold out for the next year—but fans can buy the script in book form. Within two weeks of its release, it has already become controversial among fans. The play is nevertheless required reading if you liked the novels. It’s a genuine reinvention of the Potterverse with a new cast of Hogwarts students who create very different alliances than their parents’ generation did.

Warning: Spoilers ahead!