From the introduction:
At the start of the 21st century, the powerful Tyrell Corporation designed the Nexus 6 to be virtually identical to a human but superior in strength, agility, and intelligence. This was called a Replicant. They were sent as slaves into the colonies, on risky exploration and colonization of other
After a bloody rebellion by a group of Nexus 6 in a space colony, the Replicants were declared outlaws on the earth, under penalty of death. Special police brigades, the Blade Runners, have orders to eliminate all who have not complied with the ban.