Private investigator Tom Welles is approached by Daniel Longdale, the attorney representing the wealthy widow, Mrs. Christian, whose husband has just passed away. As she begins to clear out her late husband's safe, combined with Longdale's assistance, they stumble upon a chilling 8mm film that appears to showcase a gruesome real murder. Despite the implications of the film, Mrs. Christian is determined to uncover the truth behind it.
In his quest for answers, Tom investigates missing persons files and soon discovers the identity of the murdered girl, Mary Ann Mathews. Driven by a sense of duty, he meets her mother, Janet Mathews, to discuss the horrifying potential truths about her daughter's fate. During an intrusive search of the Mathews’ home, he uncovers Mary Ann's diary, which reveals her aspirations of becoming a film star in Hollywood. Approaching Mrs. Mathews delicately, he poses a harrowing question: does she want to confront the truth, no matter how disturbing it may be? With a heavy heart, she affirms her desire to know what happened to her beloved daughter. After reading the diary and finding an accompanying note meant for her mother, he leaves the documents with her, providing her a glimpse into Mary Ann's world before departing.
As Tom ventures into the dark underbelly of the adult film industry, he enlists the help of Max California, an employee at a dubious adult video store. Together, they navigate the seedy realm of illegal pornography that leads them to Eddie Poole, a disreputable talent scout, and eventually to the violent Dino Velvet, a director known for his grotesque, masked films starring a sinister figure known only as "Machine." To deepen his investigation and gather concrete evidence, Tom decides to masquerade as a potential client eager to fund a hardcore bondage film directed by Velvet, who surprisingly agrees to a meeting in New York City.
However, the situation shifts dangerously when attorney Longdale joins the meeting, revealing that the late Mr. Christian had contracted the production of a snuff film. It's during this terrifying revelation that Tom realizes he has stepped into treacherous waters. As a means of coercion, Velvet and the masked Machine abduct Max and abuse him to manipulate Tom into producing the only existing copy of this incriminating film. In a desperate twist, Tom hands over the film but watches helplessly as they brutally murder Max and assault him before setting the film ablaze. In a final attempt to survive, Tom shocks them by disclosing that Mrs. Christian had paid a staggering $1 million for the film and hints at the involvement of Longdale, who pocketed the lion's share. This sparks a chaotic fight that results in the deaths of both Velvet and Longdale, while Tom manages to inflict injuries on Machine before making his precarious escape.
Devastated yet resolute, Tom contacts Mrs. Christian to recount his harrowing discoveries, urging her to go to the authorities. Tragically, upon arriving at her estate, he learns that Mrs. Christian has taken her own life upon receiving the devastating news of what transpired. Among her last belongings were envelopes directed to the Mathews family and to Tom himself, filled with the rest of his payment and a poignant note imploring him to "Try to forget us."
Reeling from this string of tragedies, Tom's sense of justice intensifies, prompting him to pursue retribution for the slain girl. Tracking down Eddie, he forces him to revisit the scene of the crime, grappling with the morality of his actions. When Tom calls Mrs. Mathews to reveal the truth about her daughter, he seeks her permission to deliver punishment to those responsible. Following her consent, he brutally murders Eddie with his own pistol before incinerating the evidence in his car, a haunting act of vigilante justice.
His pursuit of vengeance not yet over, Tom locates Machine and confronts him in his home. Upon unveiling Machine’s real identity as George, he is struck by the banality of evil; George, a seemingly ordinary man, coldly states, “What did you expect? A monster?” In a gripping struggle that ensues, Tom manages to kill him, finally putting an end to the cycle of violence.
Ultimately, as he returns to his family, Tom receives a heartfelt letter from Mrs. Mathews, expressing her gratitude and articulating that she believes only she and Tom truly cared for Mary Ann, serving as a somber reminder of the brutal realities behind their journey for truth and justice.
