The Underworld story, stated simply, concerns the “secret” war waged for centuries between vampires and werewolves (referred to in the films as Lycans).
The series focuses on Selene (Kate Beckinsale), a Death Dealer, who is a vampire who specializes in killing Lycans.
Selene also discovers that the leader of her vampire coven, Kraven (Shane Brolly), had lied about killing the leader of the Lycans, Lucian (Michael Sheen).
The first film, in addition to setting up the mythology, focuses on Selene hunting Lycans.