Nigerian-born Okwe, an immigrant cabdriver hustling for customers at a London airport, knows just how to chat up potential fares abandoned by their car services. "I am here," he says with elegance and precision, "to rescue those who have been let down by the system."

It is, however, one of the many ironies of the exceptional "Dirty Pretty Things" and its gripping examination of dislocation and uncertainty among that city's refugees that no one is more let down by the system than Okwe himself.

Vividly, unexpectedly, convincingly, "Dirty Pretty Things" thrusts us into Okwe's unsettling world of clandestine, quasi-legal foreigners, "the people you do not see," where the level of stability, security and accountability many people take for granted are close to unreachable goals.

It's a world where the most savage things happen to the best people, where fearful underdogs search for an angle to lift them off the bottom, where attempts at morality can seem foolish, even inhuman. An upside-down world where "there is nothing so dangerous as a virtuous man" and falling in love is the riskiest thing you can ever do.

Disturbing and intriguing, "Dirty Pretty Things" fits a great deal into its 94 quicksilver minutes. Directed by Stephen Frears from a remarkable script by Steven Knight, this is a film that insinuates itself deeply into our awareness. It's that rare pulp story with something on its mind, an unnerving, socially conscious thriller with a killer sense of narrative drive.

Those qualities flow from Knight's persuasive screenplay, the first theatrical feature from an experienced television writer who was also an unlikely creator of "Who Wants to Be a Millionaire."

With its real feeling for language and the way it is spoken by individuals as well as different immigrant groups, Knight's writing places us in the center of these uncertain lives. He also has a gift for unexpected plotting, for adding a touch of the indefinable to create a story that feels not quite like those we've seen before.

For his part, director Frears is a complete filmmaker, attracted to intelligent language as well as memorable images. Although he's had his share of misfires, Frears is indisputably one of Britain's top directors, and when he gets hold of superior material, as he has in films as varied as "My Beautiful Laundrette," "The Grifters" and "High Fidelity," he knows it. Working with top-of-the-line cinematographer Chris Menges and veteran editor Mick Audsley, Frears creates a moody, tawdry atmosphere, a London that is anonymous, uncaring, even threatening.

Desperate to survive as he faces ever-increasing obstacles is Okwe (Chiwetel Ejiofor), for whom being a cabdriver is only one of two full-time jobs. Chewing on a medicinal root to stay awake, he also works as night-shift clerk at a questionable hotel, a bizarre mini-U.N. where the doorman is Russian, the hookers multinational and his accurately named Spanish boss Sneaky (Sergi López) explains the film's title. "Strangers come to hotels to do dirty things," he tells Okwe. "In the morning it's our job to make things pretty again."

That homily is Sneaky's way of responding to Okwe's baffling discovery of a vital organ blocking the toilet in one of the hotel rooms. Although he's guarded about his personal history, Okwe is a former doctor, and while his innate sense of decency pushes him to investigate and to try to help, his questionable status and something dark in his past ("It is an African story" is all he'll say) limit what he feels he can safely do.

A similar dynamic animates Okwe's relationship with Senay (French actress Audrey Tautou), a Turkish immigrant and fellow hotel worker whose couch he chastely sleeps on. Although Okwe has something of a crush on her, Senay, timid and terrified of seeming promiscuous, tries her best to not so much as talk to him.

This tenterhooks relationship becomes even more complex when British immigration enters the picture and Okwe looks deeper into what goes on in the hotel after hours.

With its combination of familiar and unfamiliar actors, "Dirty Pretty Things' " excellent cast facilitates our bonding with the characters. A British actor whose Nigerian parents helped him with his accent, Ejiofor uses stillness and interior strength to give a powerful presentation, while López, memorable in the French film "With a Friend Like Harry," brings oily panache to his role as the ultimate fixer. As for Tautou, she delivers a major emotional performance in a role that is the complete opposite of her breakthrough work in "Amélie."

The protective coloration of immigrant communities provides a welcome anonymity these characters count on, but that's not always a reliable strategy.

In a film that takes no one's happiness for granted, it is not at all sure that there is room for both love and survival. What does it mean finally to attempt to do good in this kind of a corrupt, compromised society? When a character demands of Okwe, "Stop acting like you've got a choice," it's a measure of the integrity and individuality of "Dirty Pretty Things" that it is far from clear whether he does.