The 2,000-mile border that separates Texas and Mexico is the busiest international boundary in the world. On the United States side, Customs and Border Protection officers have the job of keeping goods and people flowing in both directions while making sure that guns, drugs and other contraband don't make it across the border. The challenge is daunting: on one side is the ruthlessly-efficient Mexican drug cartels; on the other, the biggest - and most lucrative - drug market on earth. Borderforce: America's Gatekeepers follows CBP officers stationed at 12 of the 28 international bridges that straddle the border and documents the immense challenges that the officers face on a daily basis.