露西（Lucy）是一個25歲的美國女子，居住於台北市，被男友查理脅迫運送公事箱給名為張的韓國黑幫老大， 後來她的身體被當成運毒工具，無意間吸收了半包寶貴的合成藥物CPH4（某種生物酶，類似四氫生物蝶呤）， 致使獲得越來越強大的和增強的生理和心理能力，心電感應，隔空取物，心理時間旅行，而她可以選擇不感到疼痛或其他不適。 露西前往附近的醫院得到的從她的腹部取出毒品的袋子。袋子被成功取出，通過藥物的揮發性質的操作醫生，傳感她成長的身心能力， 後來露西返回晶華酒店，襲擊了他和他的保鏢，控制生體電流強行讀取黑幫老大的記憶，從他的腦海中找到持有藥物的另外三人的位置。
Lucy is a 25-year-old American woman living and studying in Taipei, Taiwan. She is tricked into working as a drug mule by her new boyfriend, whose employer is a Korean mob boss and drug lord named Mr. Jang. Lucy delivers a briefcase to Mr. Jang containing a highly valuable synthetic drug called CPH4. 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 sales in Europe. While Lucy is in captivity, one of her captors kicks her in the stomach, breaking the bag, releasing a large quantity of the drug into her system. As a result, she begins acquiring increasingly powerful and enhanced physical and mental capabilities, telepathy, telekinesis, mental time travel, and she can choose not to feel pain or other discomforts, in addition to other abilities. She kills off her present 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 of the volatile nature of the drug and its destructive side-effects. 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 all three drug mules from his brain.
在她的公寓，露西開始研究她的病情和聯繫一家知名的科學家和醫生，塞繆爾·諾曼教授（摩根·費里曼飾演），他的研究可能是為了挽救她的關鍵。 之後，露西提供她開發能力的證明，她飛往巴黎，需要當地的警察隊長，皮埃爾·里奧幫助，在飛機上，她的細胞開始瓦解，而無需耗費更多的CPH4。 她的力量不斷壯大，讓她能夠心靈感應癱瘓武警和成員來自韓國的販毒團，並趕緊找諾曼教授，與她同意分享一切。 張先生和他的暴徒還要奪走藥物和槍戰隨之而來的法國警察。
At her shared apartment, Lucy begins researching her condition and contacts a well-known scientist and doctor, Professor Samuel Norman, whose research may be the key to saving her. After Lucy speaks with the professor 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 was she able to prevent her total disintegration. Her powers continue to grow, leaving her able to telepathically incapacitate armed police and members from the Korean drug gang. Lucy recovers the drug and hurries to meet Professor Norman, with whom she agrees to share everything she now knows, after he points out that the main point of life is to pass on knowledge. Jang and the mob also want the drug and a gunfight ensues with the French police.
在教授的實驗室裏，露西談論時間和生活的本質，以及人們對於人性扭曲的看法。在她的催促下，露西靜脈注射剩下全部三袋半CPH4，使身體在黑色物質包裹，物質散佈於電腦及其他物體。 她將這些物件轉換為空靈狀態。下一代超級電腦將包含她對宇宙的深入瞭解。然後，她開啟通往過去的時空之旅，最終來到被暗示出是露西的人類最早祖先，還跟祖先觸及指尖。 與此同時，回到實驗室時，張用AT4反坦克火箭筒摧毀大門，入內後用槍指著露西的後腦勺，打算殺了他。他開槍了，但子彈射出的瞬間，露西的腦容量用到100%，消失在時空連續體裏。她解釋，一切的關聯和存在，僅能通過時間來證明。唯有她的衣服和那外形怪異的黑色超級電腦留了下來。里奧進來實驗室，快速地殺了張。諾曼教授這時看見，拿起了由露西的身體創造的超級電腦裡頭的一體式快閃式行動碟，隨後黑色超級電腦化為塵埃消失。 里奧詢問諾曼教授露西的去向，里奧的手機隨即收到一條簡訊：「我無處不在。」
In the professor's lab, Lucy discusses the nature of time and life and how people's humanity distorts their perceptions. At her urging, she is intravenously injected with the contents of all three remaining bags of CPH4. Her body begins to metamorphose into a black substance, spreading over computers and other 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, and touches fingertips with her. Meanwhile, back in the lab, after an M136 AT4 destroys the door, Mr. Jang enters and points a gun at Lucy's head from behind, intending to kill her. He shoots, but in an instant before the bullet strikes, Lucy reaches 100% of her cerebral capacity and disappears within the space time continuum, where she explains that everything is connected and existence is only proven through time. Only her clothes and the oddly shaped black supercomputer are left behind. Del Rio enters and fatally shoots Mr. Jang. Professor Norman takes a black, monolithic flash drive offered by the advanced supercomputer created by Lucy's body before the machine disintegrates to dust. 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 you can do with it."