Deckard's boss, Bryant, tells him that they have four replicants on the loose in L.A. He explains that they killed a bunch of people and hijacked a shuttle to come to earth. Deckard's assignment is to track the replicants down and "retire" (kill) them.
When Deckard tries to suggest that Holden take the job, Bryant explains that he's already been shot and hospitalized. Deckard has no choice but to accept the mission.
Bryant fills Deckard in on some more details, showing him the video of Leon attacking Holden and explaining the replicants' names: there are two women, Pris and Zhora, and two men, Leon and Roy Batty, their probable leader. They all have a limited four-year lifespan.