A well-heeled suburbanite, Fred Morrow (Jeffrey Hunter) stops in a dismal East Los Angeles tavern to make a phone call. There, he witnesses a brutal stabbing and discovers that none of the other onlookers are willing to testify. Regardless of the consequences, Morrow decides to appear in court, but it is not long before he regrets his noble convictions. The alleged assailant, known only as Cowboy, is none-other-than Dennis Hopper and under his guidance a bevy of hoods begin terrorizing Morrow's family. Key Witness hurtles forward, following the deranged path of delinquent vengeance, a path strewn with all the necessary cultural icons-switchblades, sports cars, dope and sex. Hopper is a menacing figure, passionate in his twisted sense of justice, a charismatic leader of thugs. He flails out at a hazy, wrongful world, but his thinking is as clear as the smog-ridden sky above his head. As Variety noted, "The script doesn't attempt to explain the motivations of individual members of the gang, but the very mystery this leaves in the viewer's mind adds to the sense of terror-unrestrained by any recognizable values and hyped on dope, the gang could do anything." The darkness of collapsed urban ideals is captured grittily in Harold Wellman's energetic East L.A. location shooting.