The familiar adage about how humans only use 10 percent of their brains is an old wives’ tale (old scientists’ tale?), but that doesn’t mean it can’t be the basis for a good story. I mean, it’s not true that getting bitten by a radioactive spider gives you the powers of a spider, either, but you go with it. So what sorts of things could we do if we unlocked 100 percent of our mental potential? The possibilities are staggering!

Except in “Lucy,” that is, where the possibilities are disappointing. This weightless hunk of Euro-tech-cheese from loopy French writer-director Luc Besson (“The Fifth Element”) asks what we might be capable of if we used all of our brains, then gives this answer: “Anything! Whether it makes sense or not! As long as it fits the needs of the scene! We’re just makin’ it up as we go.”

I’ll say this for Besson, when he makes nonsense, at least it’s streamlined, fast-paced nonsense. The 90-minute “Lucy” begins with the title character — an American woman (played by Scarlett Johansson) living in Taiwan for unnamed reasons — being roped into some kind of shady deal by a sketchy guy she knows, Richard (Pilou Asbaek), a deal that involves being handcuffed to a briefcase (never a good sign).

Before you know it, a ruthless gangster named Mr. Jang (Choi Min-sik) has her and a few other poor saps smuggling an exotic new drug out of the country. But Lucy inadvertently ingests some of it, and you know what that means: now she has the powers of a spider.

No, actually, now the other 90 percent of her brain is starting to wake up, this being the desired effect of the drug. Like many of us when we are first awakened, Lucy’s brain is cranky and focused on revenge.

Meanwhile, in France, Prof. Morgan Freeman is lecturing a college class about brains, and how great brains are, and hoo boy, what if we could unlock that other 90 percent, am I right? He helpfully tells us what would happen at, say, 20 percent, and then Lucy (who might as well be in a different movie at this point) manifests those skills. Lest we misunderstand what’s happening, “20%” appears on the screen in giant characters.

Besson indulges in lots of hand-holding like that. Early scenes are intercut with snippets of stock footage that serve as metaphors: a cheetah circling a gazelle as Mr. Jang’s goons approach Lucy, for example. There are also moments when Morgan Freeman, ever the narrator, describes what’s happening for people who are standing there with him and can see it as well as he can.

Johansson is well-cast as a sexy, smart, butt-kicking femme fatale, and to wallow in goofy sci-fi malarkey is certainly not without its pleasures. This one would have been considerably less goofy, though, if Besson had taken the concept of expanding one’s brain power to logical conclusions rather than illogical ones.

Thanks to the drug, Lucy can speed-read, retain information, and remember minute details of everything that has ever happened to her. That makes sense; it’s tapping into mental agility she already had. But then she develops psychic powers, able to call a person on the phone and, just by speaking to him, see the contents of the room he’s in, or to know a person’s thoughts just by touching him or her. She also has telekinesis; she can make an invisible forcefield in the air; she can control electronic equipment; she can reshape her body; I think maybe she can even travel through time?

(She can’t fly or teleport, though. Don’t be ridiculous.)

What’s the connection between increased brain activity and superpowers? Beats me. I’m happy to accept the rules of your sci-fi world, but you do have to tell me what those rules are. Give us a few lines explaining, even improbably, how unfettered access to our brains would mean the ability to see a red pen on a man’s desk thousands of miles away. What hurts “Lucy” is the Superman conundrum: she’s all-powerful, and you can’t kill her. Besson had a good premise, but then he only used … well, he used more than 10 percent of it. Let’s be fair. Fifty or sixty percent.

C (1 hr., 30 min.; R, a lot of shooting violence, some grisly images.)

Originally published at GeekNation.