Private investigator Tom Welles is contacted by Daniel Longdale, attorney for wealthy widow Mrs. Christian, whose husband has recently died. While clearing out her late husband's safe, she and Longdale found an 8mm film which appears to depict a real murder, but Mrs. Christian wants to know for certain.
After looking through missing persons files, Tom discovers the girl is Mary Ann Mathews, and visits her mother, Janet Mathews. While searching the house with her permission, he finds Mary Ann's diary, in which she says she went to Hollywood to become a film star. He asks Mrs. Mathews if she wants to know the truth, even if it is a horrible one. She says that she wants to know what happened to her daughter, so after reading the diary and a note left for her mother inside of it, he leaves it for her and then leaves.
In Hollywood, with the help of an adult video store employee called Max California, Tom penetrates the underworld of illegal pornography. Contact with a sleazy talent scout named Eddie Poole leads them to director Dino Velvet, whose violent pornographic films star a masked man known as "Machine". To gain more evidence, Tom pretends to be a client interested in commissioning a hardcore bondage film to be directed by Velvet and starring Machine. Velvet agrees and arranges a meeting in New York City.
At the meeting, attorney Longdale appears and explains that Christian had contracted him to procure a snuff film. Longdale says that he told Velvet that Tom might come looking for them. Realizing that the snuff film was authentic, the private eye knows he is at risk. Velvet and Machine produce a bound and beaten Max, whom they abducted to force Tom to bring them the only surviving copy of the illegal film. Once he delivers it, but before he turns it over, they kill Max and beat Tom and then burn the film. As they are about to kill Tom, he tells them that Christian had paid $1 million for the film and that the reason Christian wanted the film made was for the simple reason that he had enough money to make it possible. Velvet, Poole, and Machine received much less and that Longdale kept the major portion. In an ensuing fight, Velvet and Longdale are both killed; Tom wounds Machine and escapes.
He calls Mrs. Christian to tell her his discoveries and recommends going to the police, to which she agrees. Arriving at her estate, Tom is told that Mrs. Christian committed suicide after hearing the news. She left envelopes for the Mathews family and Tom: it contains the rest of his payment and a note reading, "Try to forget us."
Tom decides to seek justice for the murdered girl by killing the remaining people involved. Tracking down Eddie, Tom takes him to the shooting location and tries to kill him. He calls Mrs. Mathews to tell her about her daughter and asks for her permission to punish those responsible. With that, he returns and beats Eddie to death with his pistol, before burning his body and pornography from his car. Tom traces Machine and attacks him at home. Tom unmasks him, revealing a bald, bespectacled man named George. He says, "What did you expect? A monster?" George goes on to tell Tom that he has no ulterior motive for his sadistic actions; he does them simply because he enjoys it. They struggle, and Tom kills him.
After returning to his family, Tom receives a letter from Mrs. Mathews, thanking him and suggesting he and she were the only ones to care about Mary Ann.