Author J.K. Rowling surprised "Harry Potter" fans this morning by announcing a new stage production based on the blockbuster series will debut next summer in London.

Called "Harry Potter and the Cursed Child," the script is a collaboration between Rowling and Jack Thorne, who adapted "Let the Right One In" for the stage. John Tiffany ("Once") will direct, with Imogen Heap writing the music.

The author's announcement arrived on the 18th anniversary of the U.K. release of the first Harry Potter novel, "Harry Potter and the Philosopher's Stone."

Rowling stressed that "Cursed Child" is a new story, but resisted revealing details. "I don't want to say too much more, because I don't want to spoil what I know will be a real treat for fans," she wrote on Twitter. "However, I can say that it is not a prequel!"

But why didn't Rowling, who's writing the "Harry Potter" movie spinoff "Fantastic Beasts and Where to Find Them," simply publish a novel? She's glad you asked: "To answer one inevitable (and reasonable!) question - why isn't #CursedChild a new novel? - I am confident that when audiences see the play they will agree that it was the only proper medium for the story."

"Harry Potter and the Cursed Child" will premiere in summer 2016 at the Palace Theatre. Tickets will go on sale this fall.