Premise: The Underworld franchise is set against the backdrop of a secret war between vampires and Lycans (werewolves who can shift between human and wolf form), that has raged for generations. The primary protagonist is Selene (Kate Beckinsale), a vampire assassin ("Death Dealer") who begins to doubt her allegiance to her side of the battle.
Warning: spoilers ahead!
While hunting a pair of Lycans, vampire assassin Selene discovers their interest in a human named Michael. She rescues him from Lycans who are testing his blood for a secret scheme, but he's bitten by Lycan ruler Lucian in the process. As she grows close to Michael, Selene discovers that vampire Kraven has forged a secret deal with Lucian to kill elder Viktor and other leading vampire figures so Kraven can rule, with Lucian receiving some of the spoils. But unbeknownst to Kraven, Lucian secretly wants to turn Michael into a hybrid and was testing his blood to see if it holds the genetic anomaly that will allow his body to contain both the vampire and Lycan virus. When Kraven finds out, he kills Lucien, but not before the Lycan convinces Selene to bite Michael to turn him into a hybrid. Fearing a mixing of the species, Viktor attempts to kill hybrid Michael, but Selene, after discovering that Viktor killed her family centuries ago -- and not Lycans, as Viktor had told her before adopting and turning her -- kills her surrogate father. As a result, she and Michael must go on the run from both vampires and werewolves. Meanwhile, the remaining elder, Markus, is awakened when the blood of a dead Lycan seeps into his sarcophagus, turning him into a hybrid.
Picking up where Underworld left off, Selene takes Michael to a safe house and returns to the vampire lair to find that Markus has turned into a hybrid and has killed Kraven. Markus hunts Selene and Michael, who have become lovers, in an effort to obtain a pendant that's the key to unlocking the prison that, on the now-deceased Viktor's' orders, has held his werewolf brother William for centuries. It turns out that Markus was the first vampire and William was the first werewolf, and they plan on taking over the world by populating it with hybrids. Selene and Michael track down the brothers' father, Alexander, and although Markus subsequently kills him, Alexander gets Selene to drink his blood, giving her hybrid-like power -- including being impervious to sunlight. Selene then kills Markus, and Michael kills the newly freed William.
This prequel explores the origin of the Lycans. Until the 14th century, werewolves were perpetually in wolf form, but one day Viktor discovers a werewolf child, Lucian, who can transform into a human. Viktor raises the boy as his slave and builds a race of Lycan slaves who guard vampires during the day while they sleep (Being part human, they'e more easily trained and domesticated.). Unbeknownst to Viktor, Lucian is having an affair with his daughter Sonja, and after leaving his post to save her from werewolves, Lucian is tossed into prison. This is the last straw in Lucian's already restless slave life, and he and other prisoners organize an escape -- with Sonja's help. Viktor finds out about their affair and the fact that she helped Lucian escape and imprisons his daughter. Lucian returns to rescue her, but he's recaptured. When she reveals that she's pregnant, Viktor puts her to death by exposing her to sunlight in front of Lucian. Lucien calls out to his Lycans in the surrounding woods, who come to his rescue and defeat the vampires in the coven. The movie ends with audio from the scene from Underworld in which Kraven tells Selene that Viktor murdered her family but spared her because she reminded him of the daughter he'd put to death.
After being held in a coma-like state for 12 years, Selene escapes imprisonment to find herself in a world where humans have discovered the existence of both vampire and Lycan clans and are conducting an all-out war to eradicate both species. She encounters a young hybrid girl who may be the key to ending the war.