I would say that the themes in the movie are freedom/captivity, trust/mistrust and selfishness/selflessness. It was a movie where aliens, called "prawns" (because they looked like prawns) found their way to earth only to find their mothership malfunctioning, and unable to get on home. Humans intervened, giving them a temporary shelter here in the planet, called District 9, sort of a refugee camp, which later turned into a slum. Aliens were acting like homeless the people that they are, started stealing, forming gangs, etc. Until finally, the humans just got tired of the violence and the hassle, and they decide to relocate them. Wikus (pronounced Vikus), is the man in charge of the relocation.
The story escalates when apparently, three prawns, one child and two adults, were actually looking for a means to go back to the mothership and fix it. But to do this, they need a kind of alien fluid that they use for fuel. After 20 years, they finally find enough to fuel them back to the mothership. The fluid was stored in a cylinder. Wikus finds this cylinder as they were serving the eviction notices of the aliens and accidentally sprays it on himself, slowly turning him into an alien, too.
The leader of the alien group who wants to escape tells him that he can fix Wikus's condition. But he needs the cylinder back (taken into a lab after they took Wikus in for testing, after which he escaped) because the equipment he needs is in the mothership. The two work together to take it back, but when the alien saw that labs where other prawns are being experimented on, he knew that he had to save his people first. He confronts Wikus and tells him of the change in plans, and he might need three years before he can fix Wikus (he needs all the fuel to get back home, it won't be enough if he'll use it to treat Wikus, too). At first Wikus is enraged, as half of his body is already alien after only 70+ hours of exposure, what more three years? His actions that follow almost destroy the alien's plan to go back home. But some gangs intervene and Wikus makes a decision to help the alien get back home. The alien, though, still promises that he will return to treat him, make him normal again. Story ends with accounts of Wikus's family, saying that since they do not know where he is anymore, they consider him dead. Just before the credits roll, Wikus is shown as a full alien.
It was one of those cliffhangers. Which I hate. I just hope that the movie showed who really they want the good guy to be. Is it Wikus who gave the alien the chance to go, forgetting his own comfort, or is it the alien, who with all sincerity and with no trace of hate, promised to come back and treat Wikus? It sure can't be both of them.
Between all these, there are lines that cover arms, weaponry and profit. But the main thought was, at least to me was: Aliens can be more human than humans. That we can find care and sincerity in the most unlikely place.