* This review contains potential spoilers.
Here's an urban legend you've seen on screen many times before: humans use only 10 per cent of their brain capacity.
That faulty logic is an oft-debunked ruse that has underpinned many sci-fi stories, from John Travolta's Phenomenon to TV show Heroes and, more recently, the 2011 Bradley Cooper vehicle Limitless.
The obvious set-up all of those offer is, what if you could access your brain's full potential?
Here to attempt to answer that in this sub-par Luc Besson thriller is Scarlett Johansson, once again skirting around any criticism of her acting ability (also see Her, Under the Skin) by playing a character whose growing brain power means she's slowly turning into an emotionless, twitchy droid.
Despite the dodgy blueprint, Lucy starts well. A brilliantly executed opening thrillride sustains suspense and sets Johannson's character up well.
She's a wayward student coerced into becoming a drug mule for a supercharged new chemical that looks suspiciously similar to Breaking Bad's blue meth.
After Asian gangsters implant the drugs in her stomach, the bag leaks and the drugs enter Lucy's system, turning her into a supercharged-but-unstable brainiac with ever-increasing control over her mental capacity.
It's there that things take a turn for the surreal, with an increasingly wayward middle section that has expansive plot strands that need plenty of explaining by Morgan Freeman's character, Samuel Norman, a brain expert who Lucy turns to for help.
Freeman is such a believable cinematic stalwart that when he says something in that low, slow monotone of his, you believe him. He's such a calm presence within the film's chaotic set pieces, it helps suspend disbelief of the otherwise silly premise.
But even he can't save the film's final third, which sees Lucy's faulty logic wobble, unravel and then fold in on itself.
It's a head-shaking shame, because Lucy looks great, with Besson's adept eye for stunning visuals, evil characters and occasional outbursts of violence giving proceedings an uneasy sense of dread.
But the way the film ends is so annoying, it doesn't feel worthy of any attempt at explanation. Let's just say it may be the biggest sci-fi WTF? moment since Indiana Jones introduced aliens in Kingdom of the Crystal Skull.
And afterwards, you may just feel like you've shaved a few percentage points from your own brain capacity.
Cast: Scarlett Johansson, Morgan Freeman, Amr Waked, Choi Min-sik
Director: Luc Besson
Running time: 89 minutes
Verdict: Great start let down by awful ending