The Mind Flayer has come to Broadway. I don’t mean the giant, spider-like creature of Netflix’s science-fiction series “Stranger Things” — though that nasty fella is here, too — but the entire beastly play “Stranger Things: The First Shadow.” The non-musical show, which shrieked open Tuesday night at the Marquis Theatre, is supersized and monstrous. My mind? Flayed. Nothing is left unscathed. Jump-scare noises blow out your eardrums. Blinding lights and raining sparks make you crave Anna Wintour’s indoor sunglasses. And there’s so much billowing haze that on the evening I attended, a family in the front row sprinted up the aisle after five minutes as though their house had caught fire. Of course, if you’re in the market for watching (fake) animals being mutilated onstage, run don’t walk. Some of the blaring special effects in director Stephen Daldry’s dizzying and sometimes nauseating production from London are impressive, though they’re nothing you haven’t seen before. The coolest one, when a huge ship materializes magically, unfortunately happens in the first 10 minutes. Prime rib for the appetizer, lettuce to follow. Many others are theme-park-ride cheesy. At one point in Act Two, I half-expected to be sprayed by water canons. But all of the expensive visuals are in service of a throwaway play in which the real villain ain’t Vecna — it’s the writing. As you become increasingly bored of the cruelly stretched plot, in which what should’ve been a 20-minute TV flashback is padded out into a nearly three-hour schlep to the inevitable, you’re waterboarded by the stagecraft. Series writer and producer Kate Trefry’s freshman stage drama is a prequel that gives Henry Creel — the baddie from Season 4 — the Darth Vader treatment. The question: How did a well-intentioned boy become “One,” the freaky forefather of Millie Bobby Brown’s “Eleven”? Henry’s journey starts when his troubled family, including his dad liquor-guzzling Victor played by T.R. Knight, moves to Hawkins, Indiana, in 1959. As soon as the loner (Louis McCartney) arrives in the sleepy town, local pets start getting violently offed. Psychic and telekinetic Henry is a bit of a male Carrie White — Stephen King’s supernatural classroom outcast — except weirder with fewer layers and no hope. Bloodshed? About the same. Curious as to what killed the cat, Joyce (Winona Ryder’s character, played by Alison Jaye apparently with the same bangs for 30 years), Hopper (Burke Swanson) and Bob Newby (Juan Carlos) investigate the gruesome crimes like hyped-up Hardy Boys. All the while, Henry’s powers — and hormones — get supercharged. He starts crushing on Bob’s sister Patty (Gabrielle Nevaeh), and their flirtation forces the audience to endure a mystifying Vegas showgirl dance number with pink feathers. It’s the strangest thing. Speaking as a longtime fan of the Netflix series, the boy’s sad story and dull subplots surrounding it are not vigorous enough to sustain such a long sit on Broadway. And if you don’t know who Dr. Brenner or the Demogorgon are, beware the Marquis escalator. Trefry, who thankfully has trimmed about 20 minutes since I first saw it in 2023, tries to bring some lightness to the drear with a cringy play-within-a-play maneuver. Joyce and the theater kids put on a silly show, in part, to trap the animal murderer, like we’re watching “Hamlet” and not just hams. These over-excited high-school students behave like they’re in a bus-and-truck tour of “Grease.” Though our refrain is definitely not “Tell me more! Tell me more!” What lifts “The First Shadow” out of the Upside Down is the fully devoted and altogether enthralling performance from the gifted newcomer McCartney. He takes a creepy part that’s a lot of twisting and shouting and turns him into a terrifying psychological case study — a ‘lil Hannibal Lecter. McCartney, a young star, seems genuinely anguished as he writhes like an electrocuted ballet dancer. The script prevents the character from ever being a likable person, but thanks to the 21-year-old from Northern Ireland, he’s a hypnotically watchable one. But there’s only so much one actor can do, no matter how talented. The material, screechy and heartless, bares little resemblance to its warm source. Part of what makes the series work, by the way, is the alchemy of its casting. Without the original kids and their 1980s garb and sweet bond, the soul of “Stranger Things” is missing. What’s mostly left is dumb and Duffer.