From Wikipedia:

The film takes place in 1970. The Vietnam War is escalating and United States President Richard Nixon has just decided on a "secret" bombing campaign in Cambodia. Faced with a growing anti-war movement, President Nixon decrees a state of emergency based on the McCarran Internal Security Act of 1950, which authorizes federal authorities, without reference to Congress, to detain persons judged to be a "risk to internal security".

Members of the anti-war movement, Civil Rights Movement, feminist movement, conscientious objectors, and Communist Party, mostly university students, are arrested and face an emergency tribunal made up of community members. With state and federal jails at their top capacity, the convicted face the option of spending their full conviction time in federal prison or three days at Punishment Park. There, they will have to traverse 53 miles of the hot California desert in three days, without water or food, while being chased by National Guardsmen and law enforcement officers as part of their field training. If they succeed and reach the American flag at the end of the course, they will be set free. If they fail by getting "arrested", they will serve the remainder of their sentence in federal prison.[1]

European filmmakers follow two groups of detainees as part of their documentary

More relevant is the show trials the detainees are put on before being sent on their trek, which are intercut through the rest of the film, and they feel like something you would hear from a Conservative kangaroo "lock them up" court if they got their way today.

Watkins was an expert at making fake documentaries about serious social and historical subjects. This is my favorite of his films.

A couple of quotes:

You don't wanna hear my message. You spent fifty years evolving a propaganda system that'll take the truth and change it into what you wanna hear. You don't wanna hear shit that's gonna mean you might have to give up something. You don't want it. All you wanna do is sit on your fat, dividend-drawing ass and draw dividends.

Would you like for me to define what a politician is? A politician is nothing but a debater. All that you do is debate issues, you fat pig, you meathead. That's all that you are, because you are lying, sucker, you're lying to the camera, you're lying to your mama, you're lying to everybody, but every time I hear you open up your mouth, all I hear is oink, you pig. That's all I hear, oink. 'Cause you ain't got no humanity in you, 'cause you're a pig, you lying punk.