Underworld opens with a voiceover describing a secret war raging between two immortal factions: the aristocratic Vampires (the “Covenant of the Undead”) and the feral Lycans (werewolves). For over a thousand years, they have battled in the shadows of human civilization.

Underworld is a 2003 American action horror film directed by Len Wiseman and written by Gregg Morgan. The movie stars Kate Beckinsale, Michael Sheen, and Shane Brolly. The film's plot revolves around a centuries-old war between vampires and werewolves, with humans caught in the middle.

The film is credited with popularizing the "action-horror" hybrid genre and led to four sequels.

Sometime after the Great War between vampires and Lycans (werewolves), the centuries-long conflict continues in secrecy. Selene, a vampire Death Dealer, hunts Lycans and works for the Vampire Council. While investigating Lycan activity, she discovers evidence that the Lycans are being used by a human conspiracy: a powerful industrialist family, the Corvins, has engineered a hybrid line linking both species. Selene learns that humans—specifically a scientist named Marcus Corvinus and his descendants—hold the key to ending the war.

That descendant is Michael Corvin (Scott Speedman), a human medical intern. Selene is ordered to kill him but instead protects him, realizing the Lycans need his blood to create a hybrid race powerful enough to end the vampire line. As Selene and Michael flee together, she uncovers a conspiracy: her mentor, Kraven (Shane Brolly), has betrayed her coven, and the vampire elder Viktor (Bill Nighy) — whom she worships — is not the noble leader she believed.