In her husband's safe, a multi-millionaire widow finds an 8mm film in which a young girl appears to have been raped and murdered most brutally. So she and her attorney employ private detective Tom Welles (Nicolas Cage) to look at the film's authenticity and decide whether it might be an actual snuff movie or not, or whether it's a fake murder. Welles tells her that the movie must be manufactured, saying that snuff movies are " essentially an urban legend—makeup, special effects...' He agrees to look into it and decides that treating the girl in the film like a missing person and seeing what comes up will be the most successful approach.
He soon discovers the girl's identity and discovers that she is, missing and that she ran away to Los Angeles to find a home in the underground porn industry. So with the assistance of Max California (Joaquin Phoenix), a clerk at a porn store, Welles finds his way through this dark underground world. That is what this film, underground pornography, and the individuals involved in it are all about. This introduces characters who appear in and produce these diverse S&M films that range from mild beatings and whippings to pornographic depictions of children in such acts, to bestiality. He aims to find the people who have been dealing with this child, and what has happened to her.
He finds the mother of the girl, too, which becomes a fascinating side plot to the story. For no particular purpose, the film deals with her coping with the loss of her only child. She becomes one of the most compelling characters in the movie since she and Welles are the only people to whom ordinary people can relate in this film. And they are opposites, opposites of one another. She's coping with the loss of the child she left and the husband she left. He is a new father with a wife who because of his job, is only beginning to detect marital issues.
In his role, Nicholas Cage is phenomenal. In this deep dark underground world, he behaves like your tour guide and is masterful in doing so. He has a sick, twisted, killer at his hands at one point, but can't come to execute him. So he calls the family of the victim and demands authorization to do so. That is one of his pure human moments. He gets closer to insanity at other moments.
8MM is a scary, dark, intense movie that will stick with you for a long time. It has its flaws in that some of the acting is less than perfect, and for one movie, there might be such a thing as too much obscurity. But it's fine in theory and features an excellent performance by Nicholas Cage. I'm just warning you that some things you can't unsee.