The plot of this film is quite simple: the eponymous Lucy is kidnapped, forced to be a drug mule, and gains superhuman powers when the synthetic chemicals she's carrying leak into her. She uses her newfound intelligence, strength and other abilities to seek revenge and understanding.
There are so many things wrong with this film, and so few right. Perhaps foremost on the list of flaws is that it is based on a widely discredited premise: that humans only use ten percent of their brain capacity, and that accessing the additional brain matter will increase intelligence. As I acknowledge you're here for a simple film review, I'll spare you the science lecture my fingers are itching to type, but I will say that this is an urban legend that has no scientific -- or indeed, any -- basis in reality. (You can turn to Wikipedia for more information on the ten-percent myth, if you're interested).
Science fiction is one thing when it speculates about where the things science has brought to our knowledge can take us, or about things we have yet to understand. Fictionalising known science, especially in a story like this one, where there were plenty of plausible avenues could have driven the same plot, is lazy and detrimental, and a piece of work that does it hardly deserves a seat at this genre's table.
But, let's say you're not as annoyed by careless (or absent) fact-checking as I am. This first point perhaps won't bother you. Still,your generosity likely won't salvage the film for you, especially if you're there for the action. In short, it's dull, trite, and, frankly, completely inexplicable most of the time. The car chase scene, in particular, made absolutely no sense, and was probably completely preventable with a tiny bit of effort or intelligent thought. There are plenty of choreographed shoot-'em-ups, but none with any flair of originality, and few with any real necessity within the story.
Strip away the science and the action meant to underpin this film, and you're left with the actual story and the characters. Neither provide redemption. Even the main character, the one around whom the story is centered, has no development. None. Literally all we know about her is that she went to Asia, was staying with another girl (presumably a friend with whom she traveled), met a bloke named Richard and dated him a week, and has a mother she calls and a father who wasn't around to speak to when she did. That's it. No other history, very few clues as to her personality before the conflict begins, nothing. At one point she laments losing everything it means to be human, but we have no idea what being human means to her. It's hard enough to sympathise with the forced mechanical voice meant to demonstrate fading emotions, without the added problem of the character being a complete stranger.
Ultimately, the story suggests it has a message to convey about humanity's place in the cosmos, but it is so convoluted and full of shockingly large plot holes, it's hard to make out exactly what that message is. Are we at the universe's whim, or are we meant to control it? Does tapping into unused brain power give us increased intelligence and strength, or does it give us magical knowledge and powers?
It's difficult to find anything positive to say about 'Lucy', because every one I think of collapses on itself. A cast of high-calibre performers like Scarlett Johansson, Morgan Freeman and Min-Sik Choi should be absolutely stellar, but their skills are underutilised with an exceedingly weak script. The plot of a forced drug mule bent on revenge, ironically aided with the source of her terror, has the makings of a thrilling and emotional journey, but 'Lucy' is methodically stripped of emotion as, apparently, intelligence increases. In the end, it boils down to an empty Scarlett Johansson being gorgeous and badass, and that sort of objectification is a bigger turn-off than any of this film's other fatal flaws.