One of the most useful aspects of the detective genre is that it creates a believable situation for people from all classes to come together. A working-class person (the detective) gets hired by an upper-class person to investigate a crime ... in the process of the investigation, the detective must check into people and places from the highest to the lowest reaches of society.
Dirty Pretty Things doesn't feature a detective, but rather a regular guy who finds himself trying to get to the bottom of a crime. Much of the film adopts the style of a crime thriller. What is interesting in that context is how invisible the upper class becomes. The characters of Dirty Pretty Things are of the servant class, the ones who, in the words of our Regular Guy, "are the people you do not see. We are the ones who drive your cabs. We clean your rooms. And suck your cocks." Yet in this film, it's those whose cabs and rooms and cocks are being serviced that we do not see. It's like a teen movie where parents are a barely-seen but always-present problem for the youngsters: Dirty Pretty Things spends most of its time making visible those who usually go unseen.
This seems to place the film outside the traditional cross-class investigation of a standard detective story, but in fact, the upper classes are always present, even when we can't see them. All of the main characters in the film are forced to confront the possibility of compromise, and in every case, those potential compromises are instigated by the lower-class status of the characters and the dirty needs of those who would hire them.
The film's metaphors are a bit thick at times ... the literalization of the concept of selling your body is wrenching but perhaps too obvious ... and while Audrey Tautou has her fans, she's a bit miscast as a Turkish woman who will later try to pass as Italian despite having what to my ears sounds like a French accent. But overall the cast is excellent, and the result is one of Stephen Frears's finest films.