Underworld is a 2003 action horror film directed by Len Wiseman and written by Danny McBride, based on a story by Kevin Grevioux, Wiseman and McBride. Kevin Grevioux wrote the original screenplay. The film centers on the secret history of vampires and lycans (an abbreviated form of lycanthrope, which means werewolf). It is the first (chronologically, the second) installment in the Underworld franchise. The main plot revolves around Selene (Kate Beckinsale), a vampire Death Dealer hunting Lycans. She finds herself attracted to a human, Michael Corvin (Scott Speedman), who is being targeted by the Lycans. After Michael is bitten by a Lycan, Selene must decide whether to do her duty and kill him or go against her clan and save him. Alongside Beckinsale and Speedman, the film stars Michael Sheen, Shane Brolly, and Bill Nighy.
An international co-production between companies from the United Kingdom, Germany, Hungary, and the United States, the film was released on September 19, 2003. Upon its release, the film received generally negative reviews from critics, but a smaller number of reviewers praised elements such as the film's stylish Gothic visuals, the "icy English composure" in Kate Beckinsale's performance, and the extensively worked-out vampire–werewolf mythology that serves as the film's backstory. A surprise hit, the film grossed $95 million against a production budget of $22 million. The film was followed by Underworld: Evolution, released three years later, and by three other films.
For generations, a secret war has raged between vampires and Lycans, an ancient species of werewolf. After years of fighting between the two enemy races, the vampires have seemingly gained the upper hand following the death of the Lycan leader Lucian at the hands of a vampire named Kraven, who subsequently became the second-in-command to the vampires' triumvirate of leading elders. Selene, a member of an elite group of vampire assassins known as "Death Dealers", longs for revenge against the Lycans, who murdered her parents when she was a child.
During a clash with the Lycans, Selene discovers that they're looking for a seemingly ordinary medical student named Michael Corvin. After she rescues Michael and takes him under her wing, the pair find themselves pursued by a group of Lycans led by Lucian, who is still alive after all. They escape, but Michael is bitten by Lucian. Since Kraven was the only witness to Lucian's supposed death, Selene comes to suspect that he lied about killing him, and may be working with the Lycans.
Now hesitant to trust Kraven, Selene returns to her coven's mansion and prematurely awakens a hibernating elder vampire named Viktor, who is waiting for his turn to resume his place as the coven's leader. Angry at being awakened early, Viktor refuses to believe Selene's warnings about Kraven's treachery, and reminds her that his fellow elder Marcus was supposed to be awakened before him.
While Selene awaits Kraven's judgment for defying him, she binds Michael, fearing that the Lycans' bite will transform him into a werewolf when the full moon rises. As the two of them bond, she gradually tells him more about her past, revealing that Viktor adopted her and turned her into a vampire after her parents' death. Soon after, Selene manages to capture and abduct the Lycan scientist Singe, while the Lycans manage to abduct and capture Michael.
While held captive in the Lycans' lair, Michael soon learns that Lucian was once in love with Viktor's daughter Sonja, and that Viktor murdered her after he discovered their forbidden love affair. Kraven claims that Lycans were once slaves of vampires, and the war began when they rose up against them and fought for their freedom. While being interrogated by Viktor back at the vampires' mansion, a captive Singe reveals that Selene was telling the truth about Kraven's betrayal, and he reveals why the Lycans want Michael: vampires and Lycans actually have a common ancestor, and Michael is a direct descendant of that ancestor. As an heir to the legendary "Corvinus" bloodline, he carries a unique genetic strain that could allow him to become a vampire-werewolf hybrid.
In the climax, the vampire elder Amelia—the ruler of the coven—is ambushed and killed by Lycans while traveling to the coven's mansion to awaken Marcus. In the ensuing showdown between the vampires and the Lycans, Selene breaks into the Lycans' lair to rescue Michael, Kraven and Lucian turn on each other, Kraven tells Selene that Viktor was the one who really murdered her parents, and Michael allows Selene to bite him - believing that her bite will make him an immortal vampire-werewolf hybrid - while Kraven shoots Lucian, killing him.
When Viktor arrives at the Lycans' lair in the aftermath of the battle, he admits to murdering Selene's parents and killing his daughter, but staunchly defends his actions. He insists that he killed Sonja for the good of his people and reveals she was pregnant with Lucian's child, an abomination in the eyes of the two species. Lastly, he claims that he made Selene immortal out of love for her. Viktor and Michael fight, but with Michael overpowered and nearly strangled, Selene turns on Viktor and kills him with his own sword. Now enemies of both the vampires and the Lycans, Selene and Michael flee the Lycans' lair together.
Back at the vampires' mansion, Marcus—now the sole surviving vampire elder—awakens after Singe's blood seeps into his sarcophagus.