As the movie begins, we see an opening text, which explains what Nexus 6 replicants are: artificial beings that are essentially identical to humans but are used as slaves on outer-space "Off World" colonies. They're not allowed on planet earth, and if any infiltrate, special policemen called blade runners are sent to track them down and kill them.
Next, we're in a futuristic L.A., complete with burning industrial torches (reflected in a human eye) and flying cars soaring near a weird pyramid-like building.
We see a blade runner, Holden, performing a special test—the Voight-Kampff Test—on a man named Leon, in order to find out if the guy is a rogue replicant or a human. The test functions by measuring the fluctuations of one of a Leon's eyes.
Holden and Leon are in the headquarters of a company called The Tyrell Corporation, and it's November 2019 (as of writing this we have about 3 years before we get humanoid slaves exploring the reaches of space so chop chop scientists).
Things start to go pretty badly, and Leon seems distressed. Finally, he pulls out a gun and shoots Holden.
The setting changes, and now, in the bowels of this futuristic L.A., we see another blade runner, Rick Deckard, getting a meal at a sushi stand. His co-worker, Gaff, interrupts him, and insists on dragging him down to the police station, where a new assignment awaits.