It's some time after the events of UNDERWORLD AWAKENING. Death Dealer Selene (ate Beckinsale) is still fighting Lycans and the vampires who betrayed her. She does have new friend, David (Theo James), whose life she had saved. The Lycans try to capture Selene because they want to know where her daughter is but she has no idea of her location, something she made sure of in the previous film, in order to protect her.
The main villain is the vampire Semira, who wants to rule the vampires. She's all too ready to kill her own kind for her own agenda. Marius, the new leader of the Lycans (Tobias Menzies of OUTLANDER), has been injecting himself with the blood of Selene's slain lover, Michael (Scott Speedman) in order to give himself the powers of a hybrid. He believes that if he gets Selene's daughter's blood that he'll be able to create true-blood hybrids and finally end the war between the two species. So why don't they all just sit down and agree to this? Because there wouldn't be a movie with 90% fighting.
There's also a new vampire Coven introduced, of a white-haired Arctic dwelling faction that take "trips" to the other world to get enlightenment. After Selene has the big confrontation with Marius and is killed-she ends up going there and coming back like the Energizer Bunny.
As with UNDERWORLD: AWAKENING, this is a completely unnecessary sequel. I was also wondering, the entire time, why there aren't any regular humans in this world? Is it just a few hundred werewolves and vampires on the entire planet? Huh?