The Brother, stranded in a completely foreign society, manages without words to communicate to us (the audience) just how peculiar we human beings are, seen through eyes unpolluted by our society's normal filters--all handled with compassion and tenderness.
Magnificent! We have an opportunity to comprehend the insanities that we have created in our society.
The drug sub-plot at the end is a bit weak. If you like this, also see Being There, Edward Scissorhands, and Slaughterhouse Five.