The U.S.-Mexican border is one of the most permeable boundaries in the world, breached daily by Mexicans in search of work. Thousands die crossing the line and those who reach "the other side" are branded illegals, undocumented and unprotected. Crossing Over puts a human face on the phenomenon, following the exodus of the Chávez clan, an extended Mexican family who lost three sons in a tragic border accident. Martínez follows the migrants' progress from their small southern Mexican town of Cherán to California, Wisconsin, and Missouri where far from joining the melting pot, Martínez argues, the seven million migrants in the U.S. are creating a new culture that will alter both Mexico and the United States as the two countries come increasingly to resemble each other.