How is the mysterious new Game of Thrones play?: Like binge-watching HBO onstage

A Lannister always pays his debts. Broadway shows rarely do, as Andrew Lloyd Webber recently pointed out with his typical restraint.But a new play based on George R.R.Martin’s “Game of Thrones” books is a big success in the UK right now.

The entire run at the Royal Shakespeare Company is sold out, and it’s only just started previews.How is it? What is it?Called “The Mad King,” the show is a prequel to the novels and massively popular TV series.It’s four hours long. Oof.

If the Night King was a character, the title could be “The Iceman Cometh”! However, rather than being set centuries earlier — a la “House of the Dragon” — the play focuses on younger versions of “GoT” favorites. Ned and Catelyn Stark, Jaime and Cersei Lannister, Robert Baratheon, Elia Martell and more spar while Daenerys Targaryen’s batty dad Aerys (the Mad King) rules over Westeros.There’s a big cast of 32 actors, about seven less than the giant “Harry Potter and the Cursed Child.”A spy — let’s call him the Master of Whispers — who caught a preview in Stratford-Upon-Avon this week was unimpressed. “There’s literally no reason for it to be a play,” he said.“It plays like a spec script for an HBO prequel and spends too much time treading water in favor of exposition.”Yet he had high praise for Michael Schaeffer’s compelling performance as Aerys and said the costumes and animal and dragon puppets are stunning.

There’s cool jousting, too, like you’re at Medieval Times without a turkey leg. “But it’s the only thing that’s innately theatrical about the show,” he said.“This is like binge-watching HBO onstage.”Of course, the RSC has had good luck with sprawling literary adaptations in the past, like “Nicholas Nickleby” and the musical “Les Miserables.” Many critics underestimated the appeal of the latter when it debuted. “The Mad King”’s director, Dominic Cooke, is highly regarded.

And the play is written by Dun...