Melancholia is a movie about a big planet that's called Melancholia. There are two sisters in the movie, Justine and Claire. Justine is very sad and she's getting married at the beginning of the movie but she's not happy. Meanwhile, Claire is trying to help Justine feel better.
Then, they see Melancholia in the sky and people start to worry about it because they think it might crash into Earth. Justine starts to feel better because she thinks that the planet is going to destroy Earth and this makes her happy because she's a bit weird like that.
As the story progresses, we see Justine and Claire dealing with their fears and emotions as Melancholia gets closer to Earth. The movie ends with Melancholia hitting Earth and everyone dying, but Justine is happy because she feels like the end of the world is a release from the sadness and pain that she has been feeling.
So, Melancholia is a movie about a big planet that's coming towards Earth and how two sisters deal with their emotions as they face the possibility of the end of the world.