The opening crawl from the movie describes the situation as follows:
Early in the 21st Century, THE TYRELL
CORPORATION advanced Robot evolution
into the NEXUS phase — a being virtually
identical to a human — known as a Replicant.
The NEXUS 6 Replicants were superior
in strength and agility, and at least equal
in intelligence, to the genetic engineers
who created them.
Replicants were used Off-world as
slave labor, in the hazardous exploration and
colonization of other planets.
After a bloody mutiny by a NEXUS 6
combat team in an Off-world colony,
Replicants were declared illegal
on earth — under penalty of death.
Special police squads — BLADE RUNNER
UNITS — had orders to shoot to kill, upon
detection, any trespassing Replicant.
This was not called execution.
It was called retirement.
A small group of rogue and extremely dangerous replicants led by Roy Batty (Rutger Hauer) have managed to hijack a spaceship and have made it to Earth, looking for a way to extend their artificially limited lifespan.
When Blade Runner Dave Holden is heavily wounded after having confronted one of them, former Blade Runner Rick Deckard (Harrison Ford) is coerced into tracking them down and “retiring” them.
Posted in: 1. About the Film