An alien mother-ship stops in the city of Johannesburg, causing curiosity and concern among humans. The aliens stay inside,
and after a few months, because of international pressure, the South African authorities raid the ship and discover sick, dehydrated and malnourished aliens. Because of poor health and the terrible living conditions inside the damp and dark ship, the aliens (called "prawns" bay men) are settled in a camp in Johannesburg which, theoretically, would only be temporary.
The camp, called
District 9, has become a slum. Drugs (canned cat food), prostitution, arms trafficking and poverty are the routine of the place. Humans and "prawns" do not mix. After twenty years of living in extreme poverty, the aliens begin to act with violence, terrorizing humans. To end the chaos that has been caused on the streets, the government was forced to resettle nearly 2 million "prawns" in what would be called District 10, a new camp far from the city, which is actually something like a concentration camp . That is when the Multi-National United (MNU) gets to the District 9 to notify the residents that they will be evicted and transferred in 24 hours. During this operation conflicts occur, aliens are killed and weapons are seized, although most of these weapons are of alien origin and only work with the DNA of the "prawns" and not humans. These weapons are extremely powerful and MNU want to produce them for profit. The situation gets really complicated when Wikus Van De Merwe, one of the commissioners to notify the "prawns" about the resettlement (who got the job for being married to the daughter of an executive), is infected with a strange alien liquid found in one of shacks and begins to mutate. He is bound to be used in experiments of MNU, since his DNA is slowly transforming from human into alien and he can use those weapons that no other human can.
Van De Merwe then manages to escape, but not knowing where to go and who to trust, the only place he can hide is District 9. There he meets
Christopher and his son. Christopher is a "prawn" that promises to help him heal if he retrives the alien fluid, which is being kept inside the MNU headquarters. This fluid is used as fuel for the mother ship and that could solve the aliens situation, sending them home. Christopher promises to go to their planet and come back in three years to help him become human again.
Christopher and Van De Merwe break into the MNU building, recover the liquid and go back to District 9 to try to make the mother ship work again. That is when a great battle beggins between the "prawns" (and Van De Merwe) and MNU. Van De Merwe protects Christopher and his son, so they can reach the ship, by attracting the attention of snipers. Christopher can then turn on the mother ship and leave, leaving Van De Merwe in District 9 wiht the hopes that one day will return to help him.