The officers and agents of the U.S. Customs and Border Protection (CBP) scour the inhospitable landscape at one of the busiest border crossings in the country, seeking to intercept illegal entrants from the air, on the ground, and at the port of entry. Every day thousands of men, women and children attempt to enter the United States illegally -- some looking for work, some seeking a new life and others trying to smuggle drugs or other contraband. Many will do whatever it takes, often risking their lives by crossing Arizona's treacherous Sonoran Desert. Now, follow the officers and agents who struggle every day to defend this border, including: Officer Hector Navarro, part of the anti-terrorism contraband enforcement team, Agent Simon Keller, part of an 18-man all-terrain vehicle unit that navigates desert terrain too rough for cars, Officer Oscar Peru, who patrols the vast Arizona desert from a helicopter, Officer Mark Shanley, a 26-year CBP veteran who uses his hard-won experience to supplement the official manual, and Chief Rene Felix, a Nogales native with a personal mission to keep drug smugglers out of his town. (Source: National Geographic Channel)