In a futuristic society that exists 'early in the 21st century', the Tyrell Corporation has advanced robot evolution to the Nexus phase, creating artificial creatures that are virtually identical to human beings. These androids are called 'Replicants'. The Replicants were originally used for hazardous or inhumane jobs considered too dangerous or undesirable for real human beings. However, when the Replicants rose up against their human creators, they were banned from Earth. Blade Runners are trained to destroy (or "retire") any Replicants they find on Earth.
Inside the Tyrell Corporation building, a man named Holden conducts a Voigt-Kampff test on a new employee named Leon Kowalski, who reacts emotionally to a few of the questions - eventually jumping to his feet and shooting Holden down. Meanwhile, Rick Deckard sits at a sushi bar and orders some noodles. However, he cannot eat his meal because Gaff appears behind him and arrests him. Gaff calls Deckard a 'Blade Runner'. At the police station, the Inspector, Bryant, admits to having Deckard arrested so that he will come to the police station and tells Deckard that he has no choice but to help, even though he does not want to.
Bryant fills Deckard in on their problem. 6 Replicants (3 male and 3 female) have escaped from an off-world Colony. They tried to break into the Tyrell Corp, where 2 of them were killed. Now 4 remain: Leon Kowalski, as well as Roy Batty, the leader of the group, Zhora, and Pris. They are physically perfect copies of human beings, except they lack human emotions. However, in case the Replicants do develop emotions, Tyrell Corp's genetic designers have programmed them to only have a 4-year lifespan.
At Tyrell Corp, Deckard meets Rachael, who works for Dr. Tyrell. Tyrell wants to see the Blade Runner conduct the test that determines the identity of a Replicant. He asks Deckard to demonstrate the Voigt-Kampff test on Rachael, a human being. After over 100 questions, Deckard determines that Rachael is actually a replicant - but she doesn't know it. Tyrell admits this, and calls Rachael an experiment - he has given her artificial memories so that she can be easier to control.
After leaving Tyrell Corp, Deckard and Gaff investigate Leon Kowalski's apartment. Deckard removes a tiny silver scale from the bathtub and a stack of photographs from one of Kowalski's drawers. Meanwhile, Leon Kowalski and Roy Batty go to see Hannibal Chew, a genetic designer specializing in making eyeballs for Tyrell Corp. The Replicants threaten Chew because they want to know about their longevity and incept dates. However, Chew doesn't know - he tells them that Dr. Tyrell is the only person with that information, and J.F. Sebastian is the one who can guide them to Tyrell.
Deckard comes home to find Rachael waiting for him. She tries to prove that she is a human by sharing her childhood memories, and even a photograph of herself and her mother - but Deckard tells her that those memories are implants, experienced by Tyrell's niece. She cries and runs away.
Meanwhile, J.F. Sebastian, another genetic designer for Tyrell Corp, finds Pris sleeping in the garbage outside his apartment. He invites her inside and she accepts.
In Deckard's apartment, he plucks at the piano and has a fantasy about a unicorn running through the woods. Later, he closely examines the photograph from Kowalski's drawer, and discovers that there is a woman reflected in the bathroom mirror. She has a blue snake tattoo on her ear. He holds up the scale from Kowalski's apartment - a clue.
Deckard finds out that the scale came from a snake at Taffey Lewis' club in Chinatown. There, he tracks down Zhora, who has been posing as a dancer named Miss Salome. Deckard chases Zhora through the streets, killing her. Bryant then informs him that he has an additional target - Rachael, who has disappeared from Tyrell Corp. Deckard thinks he sees Rachael across the street, but she darts away. Instead, Kowalski attacks Deckard, wanting to know how long he (Kowalski) has to live. Just as Kowalski is about to poke Deckard's eyes out, Rachael shoots Kowalski through the back of the head, killing him.
Back at Deckard's apartment, Rachael asks Deckard if he has seen her file and knows how long she has to live. She feels confused because she knows her emotions are not her own, but she clearly has feelings for Deckard. They kiss.
Roy Batty shows up at J.F. Sebastian's apartment and asks Sebastian to take him to see Dr. Tyrell. Sebastian eventually agrees, and wrangles an invitation into Tyrell's bedchambers at night by winning a chess game against him. Tyrell is not surprised to see Roy Batty - who is there to ask his creator for a longer life. Tyrell claims that it is scientifically impossible and tries to make Batty feel better by telling him how bright he has shined in his short lifetime. However, Batty kills Dr. Tyrell by squeezing his thumbs into the man's eyes.
Deckard receives notice that J.F. Sebastian has been killed alongside Tyrell, and goes to Sebastian's apartment to investigate. Pris is there, pretending to be a doll in Sebastian's menagerie. When Deckard gets close to her, however, she attacks him. He eventually shoots her down. Then Roy Batty comes to the apartment and finds Pris dead. Devastated, he announces to Deckard that he is coming to get him. He grabs Deckard's hand through the wall and breaks his fingers. Deckard and Batty battle it out, eventually ending up on the roof. When Deckard is about to fall, Roy Batty pulls him up and saves him. Batty ruminates that everything he has experienced will disappear in time - like tears in rain. And with that, he expires. Deckard is done. Gaff shows up and tells Deckard it is a shame that "she" (Rachael) won't live - but who does?
Back at home, Deckard finds Rachael still alive. She tells him she loves him, and he asks her to trust him. As they are leaving the building, Deckard finds a tiny silver origami unicorn and hears Gaff's voice echo in his mind. He takes the unicorn and gets in the elevator alongside Rachael.