A circus acrobat seduces a rich midget. After she's married him, her idea is to slowly poison him to death. It doesn't work out.
Back in the day, Browning took a lot of heat for using actual circus freaks for his film. Heck, even today when they want to have circus freaks in a movie, they just use some guys in makeup and maybe a dwarf or two. Browning's approach is not only much more realistic, but it's also considerably more unsettling. The pinhead girls, in particular, are a little creepy.
At the same time, this is probably the most positive portrayal of circus freaks that I've ever seen. They're shown as good people with a strong sense of community. The freaks even get a kind of a bad-ass vibe. You mess with them; you get messed with.