Background: 1,400 years ago, Vampires had decided that the best way to ensure the survival of their species was for three Elders--Viktor, Marcus, and Amelia--to 'leap frog' through time, one ruling while the other two slept, each being awakened at their appointed time according to a never-to-be-broken covenant. 1,000 years ago, during the reign of Viktor, the oldest and strongest of the Elders, the Vampire-Lycan War began.
600 years ago, Kraven set the 'great blaze' which killed Lucian, the leader of the Lycans. Kraven brought back a brand cut from Lucian's arm as proof of his death, a deed which earned Kraven high favor with Viktor, who put Kraven in charge of their coven. The great blaze caused the Lycans to scatter. Vampire Death Dealers, armed with silver bullets and shuriken, have been picking Lycans off one by one until very few remain.
Present time: Three Death Dealers--Selene, Rigel, and Nathaniel -- follow two Lycans into a subway station in an unnamed European city. A firey gun battle ensues in which Rigel is killed, one of the Lycans gets away, and Nathaniel comes up missing. In the chaos, Selene (Kate Beckinsale) makes two discoveries: 1) the Lycans have devised an ultraviolet bullet that can kill vampires, and 2) there appears to be a large band of Lycans amassing under the subway.
Selene hurries back to alert the Vampires at Kraven's luxurious mansion that sits on the outskirts of the city, but they are more interested in the clothes they are going to wear at tonight's formal party with the coven from across the 'great ocean' in celebration of the upcoming awakening of Marcus by Amelia, the current reigning Elder. Not interested in their gaities, Selene views some digital photos Rigel took of the Lycans and notices that they appear to have been following a human.
A computer search reveals the human is Michael Corvin (Scott Speedman), an intern at a local hospital. Meanwhile, the wounded Lycan has returned to the Lycan den with the bad news that he lost the candidate he was tailing, as well as to have the silver bullets and stars dug out of his body so that he can regenerate.
Selene pays Michael Corvin's apartment a visit. While snooping, she overhears a phone message about his involvement in some "shoot out." Now convinced that she is right about the Lycans stalking Michael, she tries to interrogate him just as his apartment is raided by a pack of Lycans. While Selene holds them off, Michael escapes in the elevator, but the doors open to a face-to-face confrontation with Lucian (Michael Sheen), the current leader of the Lycans.
Selene attempts to shoot Lucian, but he is intent on biting Michael and spitting the blood into a vial which he takes to Singe (Erwin Leder), the Lycan doctor who has been researching humans with the last name of Corvin, so far with negative results. He analyzes Michael's blood and pronounces it "positive."
Selene and Michael make a getaway together in her car, but Selene has lost a lot of blood in her fight with the Lycans. She passes out, the car leaps into a river. Michael saves her, binds her wound, then passes out himself. When Selene awakens, she takes the unconscious Michael back to the Vampire's mansion, incurring the wrath of Kraven (Shane Brolly), who wants Selene to be his future queen and is jealous that she seems smitten with Michael.
When Michael finally comes to, he is face-to-face with fellow vampire Erika (Sophia Myles), who realizes that Michael has been bitten and is becoming Lycan. Erika leaps to the ceiling and starts hissing at him. Scared shitless, Michael jumps out a window and escapes the Vampire's mansion.
Although it is forbidden to delve into the past, Selene decides to do a little reading about the history of the Vampire-Lycan War, particularly the chapter on the Fall of Lucian. She notices that the Lucian in one of the pictures is wearing the same medallion that she noticed the current Lucian wearing during the fight at Michael's apartment. Could he and the Lucian that Kraven supposedly killed 600 years ago be one and the same?
Could Kraven and Lucian be conspiring together? Could the fact that the Lycans are amassing near the Vampire coven portend an attack on the Vampires? When Erika informs Selene that Michael has been bitten and is turning Lycan, Selene has heard enough. Against the covenant rules, Selene decides to awaken Viktor and ask for his guidance.
Michael has begun having hallucinations and wants to know what is happening to him. Selene takes him to their Lycan interrogation room in the city so that he can hide for the night. She also gives him the good news --he has been bitten by a Lycan, and during the next full moon (tomorrow night), he will turn into one. The hallucinations, she tells him, are memories passed to him from Lucian.
When Michael asks Selene why she cares about what's happening to him ("Do you intend to bite me, too?"), Selene explains that the lycan virus and the vampire virus are incompatible and no one has ever survived a bite by both species. When Michael asks her why she hates Lycans so much, she tells him how Lycans slaughtered her entire family and how Viktor arrived just in time to save her from the same fate.
Viktor made her vampire and then raised her like his own daughter. Selene then handcuffs Michael to a chair, gives him a gun with silver bullets, and advises him to use it on himself if she isn't back by tomorrow night. After Selene arrives back at the mansion, Viktor (Bill Nighy) has fully awakened and is being briefed by Kraven.