The movie opens at Azim's (Mina E. Mina) barber shop, where he and his nephew Ekrem (Josef Altin) murder a Russian mafia gangster who is getting his hair cut. Anna Khitrova (Naomi Watts), a midwife at a London hospital, delivers a baby girl from an unconscious and hemorrhaging fourteen-year-old. The teenager dies during childbirth and has no identification other than a diary, written in Russian, and a business card for a restaurant named Trans-Siberian. Later, Anna and her mother Helen (Sinéad Cusack) are visited by her Russian-born Uncle Stepan (Jerzy Skolimowski), who refuses to translate the diary and urges her to destroy it and not get involved. Wanting to find relatives for the new-born girl, Anna sets out to uncover the mother's identity. She visits the Trans-Siberian, owned by Semyon (Armin Mueller-Stahl), who offers to translate the dead girl's diary for her. But unknown to Anna, Semyon is a boss in the Russian Mafia or vor v zakone ("thief in law"). Outside the Trans-Siberian Anna meets Nikolai Luzhin (Viggo Mortensen), the chauffeur and assistant of Semyon's unstable son, Kirill (Vincent Cassel), a violent and abusive drunk. Above Azim's barber shop Nikolai and Kirill relieve Azim of the body of the murdered Russian gangster. It turns out that Kirill ordered the ill-advised murder. Nikolai helps make the body difficult to identify by removing the teeth and cutting off the finger tips. Nikolai and Kirill then dispose of the body by throwing it into the River Thames from the end of a narrow alley. Anna returns to the Trans-Siberian the next day with photocopies of the diary pages. Semyon asks where the original is, and where Anna lives, but Anna is sufficiently careful to refuse to reply, saying she'll return in a couple of days for the translation. But Semyon has learned where Anna works, and shows up at the hospital. He admits that the diary contains compromising information about his son Kirill, and offers to provide Anna with the address of the baby's mother's family in exchange for the original diary, while subtly threatening to harm the baby if Anna does not cooperate. Uncle Stepan grudgingly agrees to translate the diary after all. Anna learns that the girl had been lured to London with promises of a well-paid job as a singer, and then forced into prostitution. To "break her in" Semyon had raped the girl, after which she was kept in a state of heroin dependency and also fed P-pills. From this information Anna deduces that Semyon must be the father of the baby girl. Mortensen as Nikolai Luzhin Mortensen as Nikolai Luzhin Semyon learns of the murder Kirill had ordered of the rival Russian gangster and questions Nikolai, who defends Kirill, saying his job is to do whatever Kirill tells him to do. This both irritates and impresses Semyon, who tells Nikolai that from now on he is to follow his orders whenever they conflict with Kirill's orders. He sends Nikolai to get the original diary from Anna, who naively thinks she will get an address in return. Nikolai discovers that Anna has a Russian uncle, and when he tells Semyon this, Semyon orders Nikolai to kill Stepan, assuming that Stephan has also read the diary and knows too much. Semyon burns the diary, but not before Nikolai has had a chance to read some of it. Nikolai later tells Semyon that Uncle Stepan has been taken care of. Some Chechen gangsters arrive in London to avenge the death of their brother, and begin by killing Ekrem. Azim visits Semyon and explains that the Chechens will kill him if he doesn't betray Kirill to them. When Semyon learns that the Chechens do not know what Kirill looks like he decides to sacrifice Nikolai to the Chechens in Kirill's place. He promotes Nikolai to the rank of captain in "vor v zakone", and Nikolai is tattooed accordingly. Azim then takes Nikolai to a bathhouse (a convenient meeting place for mafia members, because the tattoos are exposed) and indicates to the two Chechen killers that Nikolai is Kirill. The two Chechens, fully clothed and armed with knives, attack the naked Nikolai. A violent fight ensues, and Nikolai manages to kill the Chechens, although he is seriously wounded. Hospitalized after the fight, Nikolai is visited by a Scotland Yard detective specializing in the Russian Mafia; it is revealed that Nikolai is an undercover officer of the Russian Federal Security Service (FSB - a successor to the infamous KGB) working with the British police. The Russians and Scotland Yard fear for Nikolai's life and want to pull him off the operation. Nikolai rejects this, saying that he now has been promoted and has the tattoos to prove it, and should now be able to infiltrate the Russian mafia at a higher level. Nikolai also has an idea for bringing Semyon down; DNA proof that he is the baby's father, together with the mother's age, will send him to prison for statutory rape. Anna visits Nikolai in his hospital room and asks if he knows what has happened to Uncle Stepan. Nikolai claims that he has sent Stepan to a hotel in Scotland and told him to stay there for his own safety. The police arrive at the Trans-Siberian and Semyon is forced to submit to a blood test. Semyon realizes what is happening and assigns Kirill to kidnap and dispose of the baby girl. Kirill succeeds in the kidnapping, but when Anna discovers the baby missing she recruits Nikolai's help to find her. Nikolai knows where Kirill is most likely to dispose of a body and together with Anna races to the alleyway on the River Thames. They find Kirill, holding the baby by the water, suffering a crisis of conscience. They convince Kirill to not throw the baby into the river, but to hand her over to Anna. Nikolai hugs Kirill, telling him that his father will be going to prison and they will now be in control. The story closes with Anna (and her mother and Uncle Stepan) raising the child, now several months older, and Nikolai sitting alone at the restaurant as a mafia boss, but deep in contemplation, leaving the viewer to wonder who Nikolai really is and why he did what he did.