In Lucy, Scarlett Johannson is a student in Taiwan who dates the wrong guy and ends up detained and forced to be a mule for mobsters. The blue drug, a little more indigo than the stuff in Breaking Bad, is a synthetic version of some chemicals fetuses use to grow their brain. A little is a good high, a lot is ... well, never before seen. We never know exactly why Lucy is delayed from her mule flight in a cell with nasty, abusive men who kick her in the stomach releasing the contents of the bag, but it jump starts her towards the mythical "more than 2% use of her brain". And with more brain, comes more control of one's body. She kicks ass getting out.
Yes, the idea that we only use a small percent of our brain and if we had more use, we would become gods, is a myth but this is specfic and it works here. In typical stylish Besson manner, we track the increase of dosage, the increase of brain function and all the fun that comes with it. Both frightened and inspired by what she has become, she is aware of the time clock in her metamorphosis -- if it doesn't kill her, she will definitely be trans-human by the end, by the time the metre hits 100%. She can control her body, her mind, other minds, other physical objects, see transmissions, understand... everything as the number increases. And in a trippy Kubrick-est, final experience she ... becomes.
Through all of this, Lucy is in control. Of course she is, she is more human then anyone ever has been. But so many of these stories, the expanded is always along for the terrifying roller coaster, never in full control, always fearful of where it will lead. Lucy knows that things will come to an end, a finality, so she has a plan, a goal that does not make her above humanity but its benefactor. Like the AIs in the movies of late, who become one with the technology, become everywhere and everything, Lucy expands beyond comprehension but leaves a bit of herself behind. I imagine, in the end somewhere out there, she will run into Samantha from Her.