There is a lot of incomplete and misinformation out there about this movie; I hope to help fellow viewers. Yes, it is the story of a NYC detective working on the case of first one, then more, murders of women; each has been brutally beaten to death (not knifed), and the police do not see the motive or connection between the victims - we don't either at first, but there is one. The first victim is the daughter of a congressman running for reelection who is, on the one hand, a law-and-order man, and, on the other, willing to use his power and influence to manipulate the very judicial process he proclaims to support. Our detective is forced (by evidence and circumstances) to arrest the first victim's boyfriend, and faces pressure from the black community to free him. Our detective is also up against (1) a boss who doesn't like him and kowtows to the congressman, (2) the congressman himself - the two have a mutual distrust of each other, and (3) a crooked cop working for the congressman and his henchman, who are trying to derail the discovery of the identity of the real criminal. As we begin to see why, the real criminal then focuses on our detective himself. The tension really does build up to a dramatic climax, followed by a cynical ending, demanded in the 1970's.
There are frustrations with this movie. The script and direction really, well, stink, even if forgiving it a bit for being made in 1973. The lighting and handheld camera work are terrible. The music includes a good enough theme, but it is overworked. There are some nice views of NYC locales (Shea Stadium especially). All of these aspects are covered in other reviews. My biggest frustration was in figuring out who all the characters were - what their names are, and what their roles are, separate from the attention given in most discussions about who the actors are and why they are familiar to us. So here is a guide:
James/Jim/Jimmy (NOT Tommy) Blade (John Marley) is the detective. His wife is Maggie (Kathryn Walker). His partner is Spinelli (Joe Santos). He reports to Reardon (John Schuck). Another cop is Quincy (Michael McGuire).
Powers (William Prince) is the Congressman. Steiner (Keene Curtis) is his henchman. Melinda was his daughter and is the first victim.
Henry Watson (Ted Lange) is Melinda's boyfriend, arrested for her murder. Chris (Morgan Freeman) is a militant leader demanding his release.
Frederick Peterson (Jon Cypher) is the real murderer, and he works in an office with a not-so-nice guy named Bert Freund (Peter White) and Freund's secretary Joann Connors (Karen Machon). Gail (Rue McClanahan) is her friend who looks up information for Joann.
There are other characters, but these are the main ones. I hope this is helpful.
