Alfonso Cuarón's acclaimed new film, Roma, mostly revolves around the unremarkable home life of one middle-class family and one of their domestic workers—which makes it all the more surprising when, about two-thirds of the way in, it zooms out to find this same family at the center of a historic tragedy. Though Mexicans who know their history may see the signs that the movie is about to depict the infamous Corpus Christi massacre, many Americans will be blindsided and may be left scratching their heads. For those viewers, here's a guide to understanding what's brewing quietly in the background of Roma until it erupts into the political conflict that left about 120 people dead.