A Mexican show-don't-tell documentary. It helps to read up about this one. You don't have to dig through a bunch of dissertations first, but as there is very little context given during the documentary, it's certainly handy to know what the title refers to and what the film tries to document.
The Tropic of Cancer is the uppermost latitude where the sun can be directly overhead. Where it crosses Mexico, families make their living by catching animals in the wild and selling them next to a busy highway. The film follows these families around and simply observes their daily lives.
It's an interesting topic in the sense that it shows a world that is probably unfamiliar to most people, but even at 60 minutes it does become more than a little repetitive, as most of the documentary is just people catching (and somewhat mistreating) animals. The footage is pretty rough and the context is meager, making this a pretty forgettable affair.