Characters below ‘mundane’ in dopey fantasy adventure

“Mortal Instruments: City of Bones” begins with a Brooklyn girl named Clary Fray who goes to a mysterious nightclub with a friend and witnesses a horrific killing committed by a sword-wielding assailant.

What does she do? Nothing. She doesn’t tell the club management. She doesn’t call the cops on her cellphone. She doesn’t even tell her mom about it when she gets home.

What, she just forgot?

Nope. “Mortal Instruments” forgot a basic rule of fantasy adventures, that the story and characters must be rooted in some sense of reality for the fantastic elements to work.

This movie sets a new low standard for stupid fantasy adventures featuring shallow characters who can connect with each other only by using guns, blades, magic spells and chortle-inducing dialogue.

Here’s a familiar story so badly executed that not even the characters pay attention to what’s going on in it.

Take the scene where Jace (Jamie Campbell Bower), a “shadowhunter,” saves Clary (Lily Collins) and her friend Simon (Robert Sheehan) by destroying demons dressed as New York cops, whose bodies magically deflate like humanoid balloons.

“You killed two cops!” Simon yelps to Jace.

Yo, Simon! Didn’t you notice the bodies magically deflated like balloons?

It’s almost as if director Harald Zwart (who gave us the Chinese “Karate Kid”) couldn’t decide to make “Mortal Instruments” a wacky comedy referencing “Ghostbusters,” “Star Wars,” “Harry Potter” and “Road Warrior” (for the ridiculous apocalyptic costumes) or just a seriously bad fantasy adventure.

Clary turns out to be a shadowhunter, too, except her mom (Lena Headey) never told her, and now mom’s in a Snow-White coma floating in midair inside a creepy building called The Institute.

The villainous Valentine (Jonathan Rhys Meyers) needs to find the magic Mortal Goblet, and he thinks Clary can lead him to it, precipitating a special-effects-stuffed plot that requires Clary to dress like a model for teenhooker.com to save the day.

In this fantasy world — created by novelist Cassandra Clare — mortals are not called “muggles,” but “mundanes.”

If only these derivative characters could live up to that.

“The Mortal Instruments: City of Bones”

½ star

Starring: Lily Collins, Robert Sheehan, Jamie Campbell Bower, Lena Headey

Directed by: Harald Zwart

Other: A Columbia Pictures release. Rated PG-13 for violence, sexual situations. 130 minutes