In the opening shot, we see a cell split up into multiple other cells. Then we see a photo-human in prehistoric times drinking from a lake. Lucy (Scarlett Johansson
) narrates by saying that life began billions of years ago, and ponders just what we have done with it since then.
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
). 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. 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. 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. He 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. Jang steps into the next room while his men arm themselves with guns and shields. Lucy opens the briefcase, which 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.
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. 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.
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. He essentially explains 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. Basically, if the subject doesn't like their environment, they would choose immortality and move elsewhere, but if they were happy with their setting, they would be inclined to reproduce (complete with images of every species engaging in intercourse). If, say, humans could reach up to 40%, they could manipulate matter (footage of a magic show is juxtaposed here as an example). One student asks Norman what would happen if humans reached 100%. He says he has no idea.
Lucy reawakens, with her eyes glowing bright blue. She sits upright, calmer than before. A thug comes into the room, and Lucy spread 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 all the thugs' food before leaving. She finds two taxi drivers outside, kills one and gets one of them 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, stating to the doctors that they wouldn't have been able to save him because the tumor in his brain had already spread far enough. She forces the doctors to take the bag out of her stomach. She then 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, he tells her that this is something that pregnant women produce six weeks into their pregnancy as nutrients for the fetus, and that the doctor is surprised Lucy has survived this long with it in her body.
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. With her newfound powers, she does a Vulcan mind meld into his brain and sees where the other three drug mules are going based on images of their plane tickets. She leaves, and Jang lets out a pained scream.
Lucy goes to the apartment of her friend Caroline (Analeigh Tipton
) to use her laptop. She reads all of Norman's research in a matter of seconds, then contacts him 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, now that she has reached up to 20% brain capacity. She is 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. 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.
The news about Lucy shooting the patient has spread, so as she makes her way through the airport, she changes her hair color and style. She calls a French policeman, Captain Pierre Del Rio (Amr Waked
), to inform him of the other drug mules. She then boards the plane and continues her research on the brain, typing on her laptop at rapid-fire pace. The flight attendants ask her to turn off her computer, and Lucy asks for a glass of champagne, while also telling one flight attendant to wipe her nose without looking, and the lady's nose bleeds after. Lucy is given the champagne glass and toasts "To knowledge." She sips, and then "40%" flashes onscreen. She discovers a tooth in her glass, and then she spits up three more teeth. Then her skin starts to disintegrate, in plain sight of the passengers and flight attendants. Lucy runs to the bathroom, where her fingers start to disappear, and her face begins to sort of melt. She pulls out the rest of the CPH4 and snorts it, restoring herself to normal, but she passes out.
Lucy wakes up in a hospital room, with Del Rio and a few other officers and doctors waiting, right after they have apprehended the other mules. She sits up instantly, to the alarm of a nurse. Meanwhile, one of Jang's henchmen, Jii (Nicolas Phongpheth
), comes in with more armed men to find the other mules. They take them into a room and cut the drugs out of them after killing them. Lucy leaves the room and is surrounded by armed men. With a wave of her fingers, she is able to put them all to sleep, except for Del Rio. Together, they find Jang's men, and, with her brain function now at 60%, she moves them around the hall and prevents Jii from escaping with the briefcase full of drugs by creating a sort of invisible wall. She swipes the drugs from Jii and starts to leave, but orders Del Rio to go with her. He thinks he is of no use to her, but she walks over to him and kisses him, asking him to come "as a reminder."
Lucy uses her powers to find a new location, taking over Del Rio's car and driving manically through the streets, admitting she's never driven before. Jang and Jii follow her.
Lucy and Del Rio find Norman and his colleagues in a university, where she begins to divulge all she has come to realize with her brain power now exceeding that of a normal human being. While this happens, Jang and his men infiltrate the place and start shooting. Lucy gives the rest of the CPH4 to the professors for them to synthesize into a liquid material that goes into her bloodstream. Her brain function goes to 70% and her hands start to turn into a black wormy sort of goo that attaches itself to the computers in the room, giving Lucy more energy from matter, and thereby supplying her with an insane amount of power. As her brain function continues to increase, she manipulates matter in such a way that she seems to erase everything in the room, leaving the professors standing in white space before creating some kind of new age super computer before their eyes. From outside, as Jang's men are shot down by the police, he orders Jii to do something. Jii takes a rocket launcher and blasts through the room, sending Lucy thousands of miles to Times Square. She manages to move time with a wave of her hand. She travels backwards, from early 20th century New York, to colonial times, even all the way to prehistoric times to come face-to-face with the proto - human from the opening scene. They touch fingers, sending Lucy through the cosmos and all of space and time, giving her unlimited and infinite power.
Jang enters the room and slowly raises his gun to Lucy's head. And then she reaches 100%. Her body disappears, leaving her clothes behind. Jang fires his gun at nothing. Del Rio comes in and shoots Jang to death. The black matter in the room disappears and morphs into a flash drive that lands in Norman's hands, thus expanding and completing his research. Del Rio asks where Lucy is. A message appears on his phone saying, "I AM EVERYWHERE."
The film concludes with an overhead shot of Jang's corpse while Lucy's voice says this is what can be done with life.