Lucy is a 24-year-old American woman living and studying in Taipei, Taiwan. She is tricked into working as a drug mule by her new boyfriend Richard, whose employer, Mr. Jang, is a Korean mob boss and drug lord. Lucy delivers a briefcase to Mr. Jang supposedly containing paperwork, but it actually contains four packets of CPH4, a highly valuable synthetic drug. After seeing her boyfriend shot and killed, she is captured and a bag of the drug is forcibly sewn into her abdomen and that of three other drug mules who will also transport the drug for sale in Europe. While Lucy is in captivity, one of her captors kicks her in the abdomen, breaking the bag and releasing a large quantity of the drug into her system. As a result, she begins acquiring increasingly enhanced physical and mental capabilities, such as telepathy, telekinesis, mental time travel, and the ability not to feel pain or other discomforts. Her personality also changes into a more ruthless and emotionless one. She kills off her captors and escapes.
Lucy travels to the nearby Tri-Service General Hospital to get the bag of drugs removed from her abdomen. The bag is successfully removed and Lucy is told by the operating doctor that natural CPH4 is a volatile substance produced in minute quantities by pregnant women during their sixth week of pregnancy to provide fetuses the energy to develop. Lucy is fortunate to have survived with such a large amount introduced into her body. Sensing her growing physical and mental abilities, Lucy returns to Mr. Jang's hotel, kills his bodyguards, assaults Mr. Jang, and telepathically extracts the locations of the three remaining drug mules from his brain.
At her apartment, Lucy begins researching her condition and contacts well-known scientist and doctor Professor Samuel Norman whose research may be the key to saving her. After Lucy speaks with Professor Norman and provides proof of her developed abilities, she flies to Paris and contacts a local police captain, Pierre Del Rio, to help her find the remaining three packets of the drug. During the plane ride, she starts to disintegrate as her cells destabilize from consuming a sip of champagne, which made her body inhospitable for cellular reproduction. Only by consuming more CPH4 is she able to hold off her total disintegration. Her powers continue to grow, leaving her able to telepathically incapacitate armed police and members from the Korean drug gang. With the help of Del Rio, Lucy recovers the drug and hurries to meet Professor Norman. Alongside Professor Norman and his colleagues, she agrees to share everything she now knows after Professor Norman points out that the main point of life is to pass on knowledge, something for which she now possesses an infinite capacity. Jang and the mob also want the drug and a gunfight ensues with the French police.
In the professor's lab, Lucy discusses the nature of time and life and how people's humanity distorts their perceptions. She tells the scientists that time is the only true measure of human life and of existence. At her urging, she is intravenously injected with the contents of all three remaining bags of CPH4. Her body begins to change into a bizarre black substance which behaves like nanites, spreading over computers and other electronic objects in the lab, as she transforms these into an unconventionally shaped, next-generation supercomputer that will contain all of her enhanced knowledge of the universe. She then begins a spacetime journey into the past, eventually reaching the oldest discovered ancestor of mankind, implied to be Lucy. She touches fingertips with her, then goes all the way to the beginning of time and witnesses the Big Bang. Meanwhile, back in the lab, after an M136 AT4 anti-tank weapon destroys the door, Jang enters and points a gun at Lucy's head from behind. He shoots, but in the instant before the bullet strikes, Lucy reaches 100% of her cerebral capacity and ascends within the spacetime continuum, where she explains that everything is connected and existence is only proven through time. Only her clothes, Louboutin shoes, and the black supercomputer are left behind.
Del Rio enters and fatally shoots Jang. Professor Norman takes a star-filled black, presumably highly advanced, flash drive offered by the advanced supercomputer, after which the computer disintegrates. Del Rio asks Professor Norman where Lucy is, immediately after which Del Rio's cell phone sounds and he sees a text message: "I AM EVERYWHERE." With an overhead shot, Lucy's voice is heard stating "Life was given to us a billion years ago. Now you know what to do with it."