One might say that Stephen Frears has returned to his roots with “Dirty Pretty Things.” The picture resembles the British director’s earlier works–the ones he made in England before beginning the Hollywood phase of his career–in linking a commentary on a current social problem with a personal drama, rather like his breakthrough film, “My Beautiful Laundrette,” did back in 1985. In this case the serious matter is the plight of illegal aliens trying to eke out a livelihood in contemporary London. But Frears and screenwriter Steven Knight don’t treat the subject in a documentary way; they make it part of an intriguing thriller with some perversely comic overtones. The fact that the director is able to juggle such apparently discordant elements so adeptly shows that he hasn’t lost his touch during his years in Hollywood.

The linchpin of the film is Okwe (Chiwetel Ejiodor), a Nigerian doctor who tries to survive by driving a cab and working as a night clerk in a mediocre London hotel while avoiding immigration authorities. (The fact that the hotel is called the Baltic may indicate its quality, especially to those who favor Monopoly.) He shares a flat platonically with Turkish refugee Senay (Audrey Tautou), a shy, retiring girl who’s a maid at the Baltic although her visa prohibits her working. The plot kicks in when Okwe is confronted one night by a plumbing problem in one of the rooms–a toilet that’s been blocked by a surgically-removed human heart. The explanation, as he discovers to his horror, is that the night manager, the superficially attractive but sleazy Juan (Sergi Lopez), is literally inducing impoverished, desperate immigrants to sell their body parts for the black market in return for cash or, more usually, forged documents. Things escalate when Senay, under pressure from the British equivalent of the INS, loses her job and, after a stint in a sewing sweatshop (where she has to suffer sexual abuse at the hands of the owner), agrees to become one of Juan’s “clients.” This compels Okwe, whose troubled past Juan has uncovered and who wants to insure than Senay’s operation will at least occur under sanitary conditions, to accept his boss’s offer to become his in-house surgeon. Okwe, however, will prove a resourceful fellow in the end, and “Dirty Pretty Things” has an outcome that is satisfyingly, if implausibly, upbeat after all the grimness and grit that’s preceded it. (One can also be thankful for the periodic moments of humor in the earlier parts of the picture–mostly involving Benedict Wong as an exceptionally helpful morgue attendant and Zlatko Buric as the Baltic’s Russian doorman.)

The cast is solid. Ejiofor comes across as a bit too ostentatiously noble–he’s in full Sidney Poitier mode–but he brings a natural dignity and sympathy to his character. Far from the giddy effervescence of “Amelie,” Tautou creates a convincing portrait of a young woman driven to the most extreme options. And once again Lopez, as in “With a Friend Like Harry” and his cameo in the recent “Jet Lag,” makes a perfectly odious villain, oozing a rancid sort of charm. The only question about this estimable actor is whether he can successfully play anything other than a smiling bastard. As is usual in Frears’s films, the supporting cast has been chosen with care and make uniformly strong impressions.

The hybrid character of “Dirty Pretty Things” does bring some problems. There are times when its suspense-movie machinations threaten to trivialize the larger subject it raises about illegal immigration. And its depiction of the exploitation the refugees suffer occasionally becomes heavy-handed, especially in the sequences involving the INS-like investigators, who are played much too broadly. But overall it lands on the positive side of the ledger in the director’s diverse and increasingly impressive canon of work.