If you haven't seen Underworld, you're not missing all that much. There's a war going on between vampires and werewolves (here known as Lycans), and the two are not supposed to mix. Indeed, Kate Beckinsale plays Selene, a death dealer who dispatches Lycans without a thought (or, it would appear, an actual reason). The film is pretty much an excuse to make Beckinsale wear skintight rubber and PVC, and stalk around looking moody. I mightn't mind so much if she had a shred of charisma but she comes across as having all the personality of a stale sandwich.
The vampires in Underworld live in a coven, a somewhat hedonistic existence where they all prowl about pouting and squabbling for position in the hierarchy, and the arrangement is presided over by a particularly ineffectual specimen named Kraven (Shane Brolly), who simpers and scowls but never quite attains the level of menace that you'd expect from a vampire kingpin. That's reserved for Bill Nighy, who pops up as Viktor, one of the vampire ancients, and he partially saves the film from complete tedium.
The real honour for Film Saviour goes to Michael Sheen, who plays Lucian, leader of the Lycans. He's altogether a more attractive character, partly because he's Michael Sheen and automatically ten times better than anyone else who stars alongside him, and partly because he infuses Lucian with the charisma and appeal that the other characters (except Bill Nighy) lack. Part of my problem with Twilight was I was expected to prefer the vampires, but the werewolves had better characterisation, and the same thing happens here. Why would I root for the somewhat self-centered vampires when the Lycans just seem a bit more 'with it'?
So Selene stalks around the city, being all moody and monotonous in her voiceover, and she tracks down Lycans with that sort of "I'm doing my duty but I'm not going to question it" determination that you get in these warrior types. She encounters Michael (Scott Speedman) who has the capability of being a vampire/werewolf hybrid, and it all gets a bit silly so I can't remember why that's important as I was too bored with what was going on to truly take it all in.
If it wasn't for Bill Nighy and Michael Sheen, there's no way on earth that I would have included Underworld in my A to Z, but as it stands, they're both excellent, and I couldn't think of anything better...