The Earth stood still in 1982 when an alien mothership appeared over the city of Johannesburg, South Africa. As the ship remained suspended ominously in the air, a group of soldiers broke in to discover a race of aliens, all badly malnourished. Humanitarian efforts immediately commenced as the aliens were relocated to an emergency settlement camp, dubbed "District 9". Unfortunately, humanity's goodwill to the aliens soon ended: in the years since District 9's founding, the settlement became a slum as the human citizens of Johannesburg became increasingly prejudicial against the aliens - referred to by the slur "prawns" for their crustacean-like appearance and perceived bottom-feeding mannerisms - and military forces cracked down on them with ever-increasing brutality.
Eventually, in 2010, civil unrest regarding the prawns reaches a boiling point as the private military contracting service Multinational United (MNU) is brought in to relocate the prawns to a new settlement, District 10, further away from Johannesburg. Wikus van der Mirwe, the son-in-law of an MNU executive, is tasked with overseeing MNU's efforts to serve eviction notices to prawns. During the operation, Wikus finds a canister filled with a strange liquid in the residence of one prawn named "Christopher Johnson". While confiscating the canister, some of its contents spray into his face.
Wikus is eventually pulled off-duty after becoming ill and suffering a broken arm during an altercation with a prawn that resisted eviction. Over time, his condition appears to worsen as he vomits black fluid and begins to lose teeth and fingernails. When he is brought into the hospital for an examination, he discovers to his horror, when his cast is removed, that his previously broken arm had mutated to resemble a prawn's arm!
Wikus is taken into custody by MNU and forced to undergo experiments, where it is discovered that he can actually use the aliens' advanced weapons - confiscated by MNU and unusable to humans, but fully functional in Wikus's hands, both prawn and human. MNU scientists are ordered to harvest Wikus's body for research, but Wikus resists and manages to escape.
Shortly after, Wikus becomes the target of a massive manhunt - ostensibly, according to the media, for contracting a highly contagious disease from having sexual relations with a prawn - and forced to take shelter in District 9, where he takes shelter with Christopher. He learns that the liquid in the canister he had confiscated is actually fuel for the dropship that was hidden beneath Christopher's shack, and that he hoped to use it to return to the mothership, which apparently has the materials needed to reverse Wikus's transformation into a prawn. Together, they make a plan to break into MNU's research facility, where the fuel is contained, and use it to return to the mothership.
First, however, they need weapons, which local warlords sell on the black market. Wikus goes to one such warlord, Obesandjo. Obesandjo, however, is less interested in selling weapons to Wikus and more interested in chopping off his prawn arm, believing in rumors that eating the body parts of prawns would grant him the ability to use their weapons. Wikus resists and manages to grab a hold of a prawn concussive blaster, which he uses to force Obesandjo to give him weapons. Obesandjo vows to get his revenge on Wikus.
Together, Wikus and Christopher break into the MNU research lab and retrieve the fuel. Christopher, however, is shocked at the barbarous experiments the facility carried out on his people. When they return to his shack, Christopher explains that he cannot simply leave his people to suffer and must return to the homeworld for aid. As such, he wouldn't be able to have Wikus's mutations reversed until he returned in three years.
Wikus, enraged, knocks out Christopher and commandeers his dropship, attempting to fly it himself to the mothership, only to get shot down by MNU forces. Wikus and Christopher are taken into custody, only to be ambushed by Obesandjo's gangsters. MNU and the gangsters fight it out as Obesandjo abducts Wikus and once more tries to take his arm.
Fortunately, Christopher's young son manages to establish an uplink between the dropship and the mothership. The mothership begins to move towards the dropship while simultaneously activating a prawn mechanized armor suit, which Wikus uses to kill Obesandjo and his gangsters and reunite with Christopher. The two fight their way back to the dropship, with Wikus holding off the MNU while Christopher returns to the dropship and uses the mothership's tractor beam to escape.
The armored suit is totaled, and Wikus finds himself at the mercy of the MNU. He is saved, however, by the intervention of the prawns, the selfsame aliens he once oppressed with his former employers. The dropship reconnects to the mothership, which finally leaves Earth, to the elation of humanity.
Since then, District 9 was demolished and the prawns still living on Earth were relocated to District 10. Wikus was never seen again, his body eventually mutating wholly into a prawn, among whom he now lives...
