To call “Lucy” science fiction is a bit of a stretch, since the film’s basic premise — a woman is given a special drug that allows her to access 100 percent of her brain, rather than the usual 10 percent — is based on a profoundly stupid platitude. Even so, the film tackles its premise in creative, unexpected ways, and though “Lucy” is totally daft, it’s a recklessly entertaining ride with undeniable momentum.

The film doesn’t waste a second of its 89 minutes, starting off with Lucy (Scarlett Johansson) being coerced to deliver a briefcase to a hotel by a sketchy man she met in a club. She’s immediately abducted and turned into a drug mule, but the bag of drugs sewn into her stomach springs a leak. As the mystery drug pours into Lucy’s bloodstream, she unlocks more and more of her brain’s potential, while the film gets dumber and dumber.

There are plenty of interesting touches of ambition sprinkled throughout “Lucy,” which make the film worthwhile. The film’s opening conversation employs a classical editing style, juxtaposing Lucy’s trip into the hotel with shots of predators hunting their next meal with equal parts of cleverness and sledgehammer subtlety. The film’s final sequence, on the other hand, is out of control, taking Lucy to some very unexpected places and defying the wildest expectations with its audacious imagery. It’s the last ending you’d expect from this type of film, and it’s a welcome surprise to Luc Besson’s throw-everything-at-the-wall approach to science fiction.

Unfortunately, for every moment where “Lucy” manages to surprise, there’s another that fails to impress. There are only a handful of memorable action beats throughout the film, but for the most part, Besson’s staging of pivotal set pieces is unremarkable. The climax of the film is the most disappointing, a muted shootout that unfolds with minimal urgency and distracts from far more interesting things happening elsewhere. Besson’s character work is also very thin, especially for Morgan Freeman’s character, who is relegated to the role of Dr. Exposition, delivering lengthy speeches about the human brain and standing around while action scenes happen.

Johansson is having a genuine moment right now, and this year alone she has done fantastic work in “Her,” “Under the Skin,” and “Captain America: The Winter Soldier.” Though “Lucy” doesn’t ask much of Johansson, she delivers on all fronts, very effective and vulnerable in the film’s opening scenes before becoming increasingly disaffected as her brain capacity extends beyond human empathy. Though Lucy’s increasing disregard for human life as the film goes on is troubling, Johansson shoulders difficult material ably, beautifully demonstrating her range and her ability to carry a film.

It’s rare for a film as unapologetically dumb as “Lucy” to be as entertaining as it is. Though action master Besson fails at what he does best, he makes up for it with the fearlessness with which he commits to this incredibly stupid premise and the insanity of the film’s ending. “Lucy” is like listening to a sixth grader who just tried cocaine for the first time: ill-advised, badly planned, but so energetic and hilariously dim that you can’t look away.