A plagiarist steals from a single source, a “genius” from many, so the old saying goes.

But that dates from the days before cut-and-paste software. So there’s no wriggle room in that adage for Joe Ballarini, author of and screenwriter who adapted the book “A Babysitter’s Guide to Monster Hunting.”

It’s “Adventures in Babysitting” meets “Monster House” with “Harry Potter” touches, “Hitchhiker’s Guide to the Galaxy” as its plot-hook and a hint of “Beetlejuice” in its villain.

Cut. And paste.

It’s a kid-friendly mash-up of limited imagination and endless exposition. Boring as all get out, in other words.

Tamara Smart stays on brand as Kelly Ferguson, aka “Monster Girl.” No, her Rhode Island (actually PLAINLY the Pacific Northwest) classmates didn’t name her that for her acting credits (“The Worst Witch,” “Are You Afraid of the Dark”). She got that label for being a girl who swore she was attacked by monsters when she was five.

Now, trapped babysitting the fraidy-cat son of her mom’s “Ice Queen” boss (Tamsen McDonough is dressed as The Ice Queen for Halloween–hilarious.), Kelly has just enough time to bond over what might be under his bed or in his closet with little Jacob (Ian Ho) when the monsters they both kind of believe are real grab him.

Gosh, and his Mom said “No scary movies, NO trick-or-treaters, and KEEP JACOB SAFE!”

Calling 9-11 doesn’t help. It’s Halloween, after all.

Lucky for Monster Girl that Riot Grrrl She Warrior Liz LeRue (Oona Laurence of “The Beguiled” and “Pete’s Dragon” rolls up on her motorbike, a baby in her backpack, to save the day. Or night.

She reluctantly introduces Kelly to The Order of Babysitters, an underworld of “Ghostbusters” blobs stealing kids, the book “Babysitter’s Guide to Monster Hunting” and the evil plot of Grand Guignol to steal enough kids so that he “run the WORLD.”

Tom Felton of Potterworld vamps up Grand G, “stealer of dreams, bringer of nightmares.” He sings, he schemes, he takes custody of Jacob and if he can just get him to doze off and have a really good nightmare…

There’s all this monster-hunting tech and all these global babysitters organized to help with the hunt (VERY inclusive), and all these “toadies,” creatures like the Shadow Monster and Vampire Rabbit, Cloud Serpants and assorted members of “The Boogie People.”

Indya Moore makes quite the impression as the Mother of all Cat Ladies.

Naturally, they have to follow clues through Brown University and a local high school party, make time with Kelly’s idea of a hottie and deal with a mean girl.

“Since the dawn of time, every Basic Girl who’s thought she’s hot has gone for the cat costume.”

Not much that even approaches a funny line in this script, alas.

The plot is more cluttered than interesting, the effects dated but passable, the kid acting is indifferent most of the time, with Smart better at playing “the smart girl” than somebody facing her demons, fearing she’s lost a little boy to the Prince of Nightmares. Laurence isn’t much better.

Felton never lets us think he’s punching the timeclock, setting a good example that the kids aren’t up to following.

Realizing this “meh” of a movie was directed by “Tank Girl” veteran Rachel Talalay is startling and sad, until you remember how much that sucked as well. But with all this “nightmare monsters” lore and tech and effects, I could totally see “Babysitter’s Guide” becoming a Netflix franchise.

MPAA Rating: TV-PG, scary bits

Cast: Tamara Smart, Oona Laurence, Ian Ho and Tom Felton.

Credits: Directed by Rachel Talalay, script by Joe Ballarini, based on his book. A Netflix release.

running time: 1:38