Stretching for almost 2,000 miles, the border between the USA and Mexico cuts across cities, canyons, rivers and deserts. 750 miles of impenetrable wire fences, concrete walls and iron barriers already exist along the border. President Trump is determined to extend the wall along this boundary to keep out the hundreds of thousands who cross into America illegally each year. We visit the San Ysidro border crossing between Tijuana and San Diego and speak to US Border Patrol officers about the difficulties they face in policing the world's busiest land border. We meet families torn apart by the fence, and parents who live in fear of being separated from their children. We meet a vigilante who's arming himself against drug and people traffickers, and migrants hoping to make the dangerous journey across the border illegally in search of a better life. We hear moving stories of love and loss as well as of hope and compassion.