As the movie begins, a renegade replicant (read: an artificially created human) named Leon attacks a blade runner (read: a replicant-killing detective) who's performing a test on him.
Turns out that Leon is one of four replicants who have escaped their lives of slave labor in space colonies to come to earth, searching for the man who created them: Dr. Eldon Tyrell. They're nearing their expiration dates, and they want to extend their lifespans beyond the set four-year limit Tyrell programmed into them. The Los Angeles Police Department enlists a blade runner named Rick Deckard to track 'em all down.
Hello, Harrison Ford.
But things quickly get murky for Deckard:
Sebastian leads Roy to Dr. Tyrell's residence, where Roy demands a normal human lifespan. Tyrell says this is impossible, and Roy murders him (and Sebastian, for good measure). Back at Sebastian's apartment, Deckard has a duel with Pris, whom he finally kills, despite her impressive gymnastics. Roy arrives and starts to flip out: he chases Deckard around the apartment but finally decides to spare Deckard's life with an unexpected act of mercy.
And Roy dies that same night.
Deckard escapes back to his own apartment, where he and Rachael reunite. They decide to run away and try to lead a new life together. Depending on the version you're watching, you get a very different ending: