A young woman enters a London pharmacy, but before she can explain what she needs, she begins bleeding and collapses. She dies shortly thereafter in a nearby hospital, leaving behind a daughter and a diary.

It might all have ended there, had not Anna, the midwife who delivered the baby, become obsessed with finding the young woman's family and, presumably, a home for the baby. So Anna (Naomi Watts) turns for clues to the diary, which is in Russian, but which contains a business card from a local Russian restaurant. And it isn't long before Anna has recruited the restaurateur, Semyon (Armin Mueller-Stahl), to translate it for her.

And there begins Anna's descent into a London subculture that most people would rather not know about, much less have to avoid: the "Vor V Zakone" (translation: "thieves by the code" or "thieves in law," a.k.a., the Russian mafia), a gang that makes the Sopranos look like Habitat for Humanity.

Just how this organization operates isn't made entirely clear in director David Cronenberg's latest film, Eastern Promises. We do learn that is revolves to some extent around Semyon and his son, Kirill (Vincent Cassel), and that the restaurant, while apparently a very good one, also serves as a front for numerous less legal operations: bootlegging, drug running and the sex slave trade.

Anna learns this, too, if a bit late, because she has a second translator working for her: her Uncle Stepan (Jerzy Skolimowski), also a Russian immigrant, though one not tied to the Vor V Zakone. With the begrudging aid of Stepan, Anna discovers the young woman (Sarah-Jeanne Labrosse) who died giving birth at the hospital -- a 14-year-old named Tatiana -- had been brought to London to work as a prostitute by and for Semyon. And the relationship didn't end there.

Just where this all leads is what makes Eastern Promises both difficult to watch and impossible not to. Eastern Promises is a brutal tale, an endless stream of bloody killings that, had you missed the beginning, might have fooled you into thinking you'd walked in on a slasher film.

What distinguishes Eastern Promises from such fare, however, is an intelligent script that offers viewers an insight into a very real world most have never heard of or know little about. Cronenberg takes us deep into this unnerving subculture, most notably in a scene where Semyon's "driver," Nikolai (Viggo Mortensen), is "interviewed" by a gaggle of gang bosses in Semyon's darkened restaurant, to see if he's worthy of promotion.

Nikolai sits before the gang bosses in only his briefs, displaying the tattoos he got in Russian prisons -- his resume, actually -- and concurs with all they say, including slurs against his parents. It's a scene that might lead viewers to despise Nikolai, if he hadn't already distinguished himself as an incredibly competent and perceptive creature, one capable of reacting calmly to the insanity and obsessions that flow so swiftly around him.

"I am driver," he tells Anna as she pelts him with questions about Tatiana. "I go left, I go right, I go straight ahead -- that's it."

All of which points to one of the real strengths of Steven Knight's script. Knight, who was nominated for an Oscar for best original screenplay for Dirty Pretty Things, creates a cast of characters who are capable of scaring us half to death one second and winning us over the next. And he rarely wastes a word -- no easy task in a film that features at least three languages (English, Russian and Turkish) and easily as many or more ethnic groups.

The result is a London that's clearly not the London of old: it's open, it's wealthy, but it's disturbing in ways even Dickens couldn't have imagined. It's overrun by people who are all too willing to, and do, slit throats in public or private to make a point, or even carve up a corpse if it brings home the message they're trying to deliver.

Add to this performances by actors like seasoned pro Mueller-Stahl, who's played everything from a Nazi war criminal (The Music Box) to the Israeli prime minister on West Wing and you have a movie that's both intelligent and perceptive -- and doesn't let you rest for a second.

Stuff like this doesn't come along very often. Make a note of it.