Underground, made by three film-makers, producer Emile de Antonio, editor Mary Lampson, and cinematographer Haskell Wexler, has been described as 'an anthology of the left'. It includes interviews with five members of a radical American protest group, the Weather Underground, which has claimed responsibility for twenty-six bombings. The fugitives call for the revolutionary overthrow of the American system, and discuss bombings in the Capitol building and the Rockefeller Centre bank. The faces of these figures are never seen on screen, the film-makers having gone to considerable lengths to protect their identity. The origins of the movement are described, especially the evolution of the Weather Underground from the Civil Rights movement and SDS activity on American campuses during the late sixties. The film uses clips from earlier documentaries, including Attica and Murder of Fred Hampton.

De Antonio's earlier films include In the Year of the Pig and Millhouse: A White Comedy, and he has been heavily attacked in Congress for his radical film-making activities. At one stage during the making of Underground, Federal Grand Jury subpoenas were issued (and later withdrawn) to hold the film already shot as evidence for investigation into the Weather Underground. De Antonio claims that he and his fellow film-makers on the project have been subject to continual FBI harassment, including wire-tapping and other forms of surveillance.