When kids invent friends to play with, they’re showing how fertile they are with their idle hours, and how resilient and efficient the brain can be. In fact, I’d bet if you tasked any child with a rewrite on this script (credited to director Jeff Wadlow, Greg Erb and Jason Oremland), you’d likely come up with something way more colorful, fun and freaky than the zipless cafeteria food ladled onto our laps.

“Imaginary” arrives in the shadow of the new date-night classic “M3GAN” (also from Blumhouse), itself a benchmark in the weaponizing of a troubled kid’s playtime, revealing in its craftiness a surprisingly rich engagement with tech-age loves and fears. “Imaginary” is marked by a similar starter kit of broken-home emotions: Earnest stepmom and storybook illustrator Jessica (DeWanda Wise) thinks the way to deepen her relationships with the glum daughters of her new husband (a negligible Tom Payne) is to move everyone into her old childhood home.

Teenage Taylor (Taegen Burns) mopes and sasses, but nervous moppet Alice (Pyper Braun) finds joy in an understuffed, red-vested bear discovered (where else?) behind a hidden door in a spooky basement. Chauncey soon exerts a remarkable, eventually menacing control over Alice, who voices what he tells her, triggering Jessica’s own repressed trauma.

DeWanda Wise, right, and Pyper Braun in the movie “Imaginary.”

(Parrish Lewis / Lionsgate)

But where “M3GAN” felt alive to the culty potential of a malevolent plaything, “Imaginary” skips the directive to entertain, coming off as stiff, pedestrian and dreary as a March space-filler can get. When Betty Buckley’s grandmotherly scholar shows up to give us a crash course in the cultural history of kids with spirit besties, an already weak film suddenly exhibits the tedium of a book report. (Since when did horror screenwriters feel the need to start explaining everything? Oh, “The Innocents,” take me away.)

Wadlow is, frankly, terrible with actors. Wise, his lead, has plenty of presence, but she never seems to be truly grappling with nightmares, tormented stepkids, a creative block, a father in an assisted living facility and the fallout of a truly horrendous family relo. The awkwardness is evenly applied: Anytime two people in the same scene are supposed to know each other, they look as if they’ve just met. Wadlow can even make a motionless plush animal seem poorly directed.

Chauncey is the least of his problems, though, when what’s been structured around those moments feels like a paint-by-numbers exercise and not a true wade into terrifying waters. When the final act shifts to a world behind a portal — remember, there’s a kid here named Alice — you feel “Imaginary” giving up any interest in shaking our senses, hoping a late injection of CGI twists will do the trick. If kids can grow out of their pretend pals, so too can horror audiences of cynical snoozes like this.