Staff members prepare to sell copies of “Harry Potter and the Cursed Child” at Foyles bookstore in London last month. (Rob Stothard/Getty Images)

This piece discusses the prose, and one detail of the plot, of “Harry Potter and the Cursed Child.”

Be careful what you wish for — you just might get it. Back in March, I wrote that it was time for J.K. Rowling, who wrote the massively popular Harry Potter novels, to start letting other people play with the ideas she originated, resulting in something like the Star Wars Expanded Universe. Five months later, the release of “Harry Potter and the Cursed Child” is making me bitterly regret writing that piece.

I want to be clear: This piece is no way intended as a review of the stage production “Harry Potter and the Cursed Child,” which I haven’t seen and am extremely unlikely to see in its current iteration. But because the script for “Harry Potter and the Cursed Child” has been released as a stand-alone volume, marketed as a new installment of the Harry Potter story, and is selling huge numbers of copies, it’s worth reviewing as a script and as a piece of writing.

And on those scores, it is a rather dispiriting failure.

I don’t know that readers tend to think about J.K. Rowling’s prose when they remember why they found the seven “Harry Potter” novels captivating. Instead, they probably dream about which house they might be sorted into, or reminisce about the complexity and riskiness of Albus Dumbledore’s plans to help Harry defeat Voldemort, or talk about Severus Snape, the embittered potions master who just might be Rowling’s greatest creation.

For me, one of the great pleasures of reading Rowling’s books was watching her fine-tune her writing, her word choice and sentence structure becoming increasingly sophisticated as Harry and his friends grew and matured.

I don’t know that she ever lived up to Aeschylus, the ancient Greek playwright from whom she chose the epigraph for “Harry Potter and the Deathly Hallows.” But rereading that book, I’m struck by little things: the use of “corrosive” to describe Harry’s feelings toward Ron during a bitter argument; the rhythm of the “blood and nerve and bounding heart” Harry notices as he walks toward his death; the simplicity of Snape’s “Always,” when he affirms that his Patronus, the expression of his greatest happiness, mirrors Lily Potter’s. The efficiency and precision of Rowling’s language in moments like these more than make up for some pacing issues in the novel, or the fact that Harry’s final confrontation with his great foe essentially turns on a long monologue.

So it grieves me to report that much of Jack Thorne’s writing in “Harry Potter and the Cursed Child” is simply not very good.

There are the cliches: “Don’t play games,” “For my sins,” “light in the darkness.” The play also has its share of lines that seem as though they’re meant to be clever but end up sounding rather clunky instead, as with a declaration from Professor McGonagall that “Your solidarity is admirable, but it doesn’t make your negligence negligible.” Developing individual voices for the characters is critically important to making a script sing, and “Harry Potter and the Cursed Child” is so caught up in plot machinations and alternate timelines that it never manages to achieve this task.

The play is also burdened by a number of moments when the barrier between the wizarding world and the one we occupy seems to have fallen, and not in any valuable way. Finding out that Moaning Myrtle’s last two names are “Elizabeth Warren,” or that in one timeline, Draco Malfoy’s son’s school nickname is “Scorpion King” feels less like one dimension echoing powerfully in another, and more like Thorne, or his collaborators including Rowling herself, couldn’t be bothered to Google and make sure their language was actually original. If there’s anything a “Harry Potter” story shouldn’t do, it’s evoke a sitting U.S. senator or a bad movie starring Dwayne Johnson.

Cringingly, there’s even a moment when one character tells another “you’re the worst spoiler in the world that didn’t turn out to be true.” That’s not really meta, just terrible.

If it sounds as though I’m nitpicking, I’m only really meeting the latest installment of the “Harry Potter” saga at the level that fans of the franchise — and Rowling herself, in invoking Greek drama — aspire to for it: that it be recognized as literature. And while you can go light on singing prose and striking character development and still clean up at the bookstore and the box office, there’s a difference between being a massive franchise and being enduring art. By the end of her novels, Rowling was stretching her writing. The language of “Harry Potter and the Cursed Child” feels like a real comedown.

Even the late-discovered prophecy that defines the action in “Harry Potter and the Cursed Child” doesn’t quite invoke the sense of wonder and terror it might have had it only been written better: “When spares are spared, when time is turned, when unseen children murder their fathers: Then will the Dark Lord return.” It’s enough to make me long for the compositions that emerged from the inanimate Sorting Hat.