America continues to wage a battle against the stream of undocumented immigrants entering the country. An estimated three million undocumented immigrants currently reside in the US. Each year, another three hundred thousand illegal immigrants arrive in the US in addition to the nearly nine hundred thousand who are legally accepted. How long can America sustain this influx of immigrants? And how real are the growing fears about economic costs and long-term social and political disruption? Frontline correspondent William Langewiesche explores these questions, focusing on California.