In the opening shot, we see a cell split up into multiple other cells. Then we see a proto-human in prehistoric times drinking from a lake. Lucy (Scarlett Johansson) narrates . . .
Lucy: Life was given to us a billion years ago. What have we done with it?
In the present, Lucy is a young American woman outside a hotel in Taiwan with her week-long boyfriend Richard (Pilou Asbæk). He is asking her to take a briefcase up to the room of a Mr. Jang (Min-sik Choi).
Richard: Lucy. It's as easy as pie. In and out, all done and dusted.
Lucy: Then why don't you do it yourself?
Richard: The last thing the guy's expecting is a total 10 turning up to deliver the case. It'll blow his mind.
Lucy: You get paid a thousand dollars for delivering paperwork?
Even though offered $500, she refuses to do anything for Richard without knowing what it is that he wants her to deliver, so he handcuffs Lucy's wrist to the handle of the briefcase, leaving her with no choice.
Lucy: Take this off me right now!
Richard: I'm sorry, I have no choice.
Lucy: You're an asshole!
She enters the lobby and tells the clerk that Richard is sending her up to Jang, and then nervously gives her name to the clerk.
Lucy looks out the window to see Richard smiling and trying to be encouraging, until he is shot and killed.
A group of thugs come out of the elevator and force Lucy to go up with them.
Lucy: Please! I don't know anything!
This scene is juxtaposed with footage of a cheetah chasing a gazelle, mauling it and then carrying the gazelle's corpse in its teeth.
They take Lucy to Jang's apartment. There are bloodied corpses on the floor, causing Lucy to vomit. Jang comes out with his hands soaked in blood.
Lucy: Do you speak English?
Jang rings up a man on the phone to translate for Lucy.
She cries, not knowing what is going on or why any of this is happening. Jang writes down number '140' on a piece of paper, which is the code to open the briefcase.
Man on speaker: I speak English.
Jang steps into the next room while his men arm themselves with guns and shields.
Lucy: What's really in here?
Lucy opens the briefcase, to her relief nothing harmful happens.
Man on speakerphone: Can you describe the contents in the case?
Lucy explains it contains four 1 kg bags of blue powdered crystals.
Jang brings in a drugged-up man to snort some of the blue powder. The man briefly convulses and then starts laughing maniacally until Jang shoots him in the head. Lucy is then ordered to do a job, which she refuses to do until she gets smacked.
As all of this happens, we meet Professor Samuel Norman (Morgan Freeman) giving a lecture to a classroom on his research of the brain's capacity.
Professor Norman: Animal life on Earth goes back millions of years. Yet most species only use 3 to 5% of its cerebal capacity. But it isn't until we reached human beings at the top of the animal chain that we finally see a species use more of its cerebral capacity. 10% may not seem like much, but it's a lot if you look at all we've done with it.
As he talks we are shown examples of man's accomplishments - first flight, fighter jets, road way, armies, robots, stock market, rockets, satellite. He goes on to explain what could happen if humans could exceed beyond the suggested 10% brain capacity that they use, as well as its responses to certain environments that would allow the brain to submit to immortality or reproduction.
Professor Norman: We humans are more concerned with having than with being.
Lucy wakes up in a hotel room. There is a bandage on her stomach. Jang's men come in and throw her some clothes. She is taken to Jang, along with three other men.
A British man known as "The Limey" (Julian Rhind-Tutt) comes in to explain to Lucy that she and the other men have had a drug called CPH4 stuffed into their lower intestines.
The drug is supposed to be the next big thing on the market, and they intend to smuggle it across the world.
The Limey: So gentlemen, and madam, allow me to be the first to wish you bon voyage.
Professor Norman: For primitive beings like us, life seems to have only one single purpose: gaining time. And it is going through time that seems to be also the only real purpose of each of the cells in our bodies. To achieve that aim, the mass of the cells that make up earthworms and human beings has only two solutions. Be immortal, or to reproduce.
If its habitat is not sufficiently favorable or nurturing, the cell will choose immortality. In other words, self-sufficiency and self-management. On the other hand, if the habitat is favorable, they will choose to reproduce. That way, when they die, they hand down essential information and knowledge to the next cell. Which hands it down to the next cell and so on. Thus knowledge and learning are handed down through time.
We are shown images of a number of species engaging in intercourse.
Afterwards, Lucy is taken to a room and chained up, where one of the thugs, tries to get into her jeans, then starts kicking her in the stomach, right where they cut into her.
This causes a tear in the drug bag, leaking the powder into Lucy's system. She begins to writhe and thrash all across the room, even reaching up to the ceiling until she falls back down to the floor.
Professor Norman's presentation continues.
Professor Norman: Let's imagine for a moment, what our life would be like if we could access, let's say, 20% of our brain's capacity. This first stage would give us access to and control of our own body.
Student: Sir, has it been proven scientifically?
Professor Norman: For the moment, it's just hypothesis, I confess. But if you think about it, it's troubling to realize that the Greeks, the Egyptians and the Indians had notions of cells centuries before the invention of the microscope. And what to say about Darwin, whom everybody took for a fool when he put forth his theory of evolution.
It's up to us to push the rules and laws and go from evolution to revolution. 100 billion neurons per human, of which only 15% are activated. There are more connections in the human body than are stars in the galaxy. We possess a gigantic network of information to which we have almost no access.
Student: Sir, and what will be the next stage?
Professor Norman: Well, the next stage would probably be control of other people. But for that, we would need to access at least 40% of our brain's capacity. After control of ourselves and others would come control of matter.
Footage of a magic show is juxtaposed here as an example.
Professor Norman: But now we're entering into the realm of science fiction and we don't know any more than a dog who watches the moon.
Student: Excuse me, sir. But what would happen if for some reason we ignore, somebody unlocked 100% of their cerebral capacity?
Professor Norman: 100%? . . . I have no idea.
Lucy reawakens, with her eyes glowing bright blue. She sits upright, calmer than before. A thug comes into the room, and Lucy spreads her legs to tempt him. The thug smiles and lays down his gun on the table. As he walks over to her, Lucy grabs his belt and flips him over, then pulls the table over to take the gun.
After she frees herself, she shoots the other thugs as they're eating. She gets shot in the shoulder, but is able to dig the bullet out without feeling pain. She then eats the thugs' food before leaving. She finds two taxi drivers outside speaking Mandarin.
Lucy: Hey, you speak English?
Taxi Driver 1: No, no.
Lucy shoots him and gets the other driver to take her to the hospital.
On her way to the hospital, Lucy has heightened senses and is able to hear peoples' voices from outside the car. When she gets to the hospital, she is able to clearly read the signs as if they were in English to her. She gets to an operating room where doctors are working on a man. Lucy looks at the patient's X-ray scans and then shoots the man.
Lucy tells the doctors that they wouldn't have been able to save him because the tumor in his brain had already spread far enough.
Lucy: Somebody put a bad drugs inside me. I need you to remove it. It's leaking.
Surgeon: Okay. Calm down.
As he operates she grabs his cellphone.
Lucy: Do you mind?
She calls her mother to tell her that she feels everything - the air, the blood in her veins, the heat leaving her body, and is even able to access the deepest parts of her memories, like when she was sick as a child or even petting a cat when she was barely a year old. She tells her mother that she loves her before hanging up. The doctor pulls the bag of drugs out of Lucy's stomach, and after she tells him that it's CPH4, she asks the surgeon to tell her more about it. He explains that this is something that pregnant women produce six weeks into their pregnancy as nutrients for the fetus.
Surgeon: It packs the power of the atomic bomb. I've heard they tried to make a synthetic version of it. I didn't realize that they'd succeeded. If it really is C.P.H.4, and this quanity, I'm amazed you're still alive.
Lucy: Not for long.
Lucy makes one more stop back to the hotel to find Jang as he is in the middle of getting a tattoo and a facial.
She kills his guards and then shoos the tattoo artist away before sticking knives in both of Jang's hands.
Lucy: Learning is always a painful process. Like when you're little, and your bones are growing, and you ache all over. Can you believe I can remember the sound of my own bones growing? Like this grinding under the skin. Everything's different now. Like, sounds are music I can understand, like fluids.
It's funny, I used to be so concerned with who I was and what I wanted to be, and now that I have access to the furthest reaches of my brain, I see things clearly and realize that what makes us us is primitive. They're all obstacles. Does that make any sense? Like this pain you're experiencing. It's blocking you from understanding. All you know now is pain. That's all you know, pain. . . Where are the others? The others carrying the drugs. I need the rest of it for medicinal purposes.
With her newfound powers, she does a Vulcan mind meld into his brain.
Lucy is able to scan his memories and zoom in on the details of the airline tickets the mules are carrying - Berlin, Paris, Rome.
Lucy: Thank you for sharing.
As she leaves, Jang lets out a pained scream.
Elsewhere, Professor Norman arrives at the Westin Hotel to retire for the evening.
Lucy goes to the apartment of her friend Caroline (Analeigh Tipton).
Caroline: You scared the shit out of me! I miss you.
Lucy: I miss you, too.
Caroline: Yeah, you lose your keys or something?
Lucy: No. Can I borrow the laptop?
Caroline: Yeah, of course. So, I spent all of yesterday in auditions, which are awesome. They just have you standing around all day because they don't give a shit about your time gabbing away in Chinese. Who understands Chinese? I don't understand Chinese. And then they say they're always going to call, and they never do, because they don't even take your phone number.
While Caroline is blabbering on, Lucy reads all of Norman's research in a matter of seconds.
Caroline: Except this one guy, but he's not Chinese. He works at the agency and he's cute. Oh my God, he's cute. You know, like in the 'cute-and-I-kind-of-know-it' kind of way. Like he had this thing. Oh my God, and his ass! Phew! Let me get started on that! And you are never going to guess where he took me.
Lucy repeats Caroline's response at the same time.
Lucy and Caroline: Four Seasons, Royal Suites, and we made love all night.
Caroline: I'm sorry, I'm talking about myself. What's up? How's Richard?
Lucy: He's dead.
Caroline: You guys are crazy. I'm going to take a shower.
Lucy phones Norman to tell him what she's read. He is astonished to hear that she read everything, even as she continues to explain what she is feeling and experiencing.
Lucy: I read your theory on the use of the brain's capacity. It's a little rudimentary, but you're on the right track. Professor, my cells are reproducing at a phenomenal speed. Several million per second. I'm having trouble precisely evaluating the time of my death, but I doubt I'll last more than 24 hours.
What I'm saying is your theory is not a theory. I aborbed a large quantity of synthetic C.P.H.4. that will allow me to use 100% of my cerebral capacity. Right now I'm at 28%, and what you wrote is true. Once the brain reaches 20%, it opens up and expands the rest. There are no more obstacles. They fall away like dominoes. I'm colonizing my own brain.
Norman: Well, I don't what to say. It's true, I've been working on this theory for over 20 years, but it's only ever been a hypotheses and research ideas. I never though any would . . . You can control your own metabolism?
Lucy: Yes. And I can start to control other people's bodies.
She is also able to manipulate electronics, making her appearance on the TV in Norman's hotel room, as well as his phone and the radio. She plans to meet with him in 12 hours.
Professor Norman: You know, if you think about the very nature of life - I mean, on the very beginning, the development of the first cell divided into two cells-the sole purpose of life has been to pass on what was learned. There was no higher purpose. So if you're asking me what to do with all this knowledge you're accumulating, I say pass it on. Just like any simple going through time.
Before leaving, she prints out a prescription for Caroline, telling her that her liver and kidneys are failing and that she needs to make new lifestyle choices and she'll be okay.