The best way I can sum it up for you: remember when you were a little kid and wanted to play Your Favorite Movie, but they didn't make toys for it so you had to use whatever action figures you already had, and then your best friend wanted to play Your Best Friend's Favorite Movie and so you wound up smushing together tons of stuff that made no sense whatsoever?
KID 2. "And then there's a castle!"
KID 1. "What? No!"
KID 2. "But look! We've got knight figures! There's got to be a castle!"
KID 1. "Okay, there's a castle. Where our heroes get captured and fight to the death!"
KID 2. "Awesome! They'll need grenades, though. To escape with."
KID 1. "Well, yeah. Of course they'll have grenades."
Basically, Neil Marshall wanted to play ESCAPE FROM NEW YORK and his buddy wanted to play MAD MAX but the kid from down the street was determined they'd play 28 DAYS LATER and his little sister just wanted to play knights, and at the same time his big brother remembered these Dave Gibbons comic stories with cannibalistic savages in postapocalyptic cities, and the kids are having fun but if you think about it too much you'll have to lie down with a headache. Or, as Jesse Custer put it on seeing an octogenarian Ku Klux Klansman sausage-maker snuggling up to a giant meat lovedoll: "I've seen a lot of fucked-up things in my time, but that about takes the fucked-up cookie."
The short version of the premise is this: a devastating virus forces authorities to quarantine Scotland, walling it off from England. About thirty years later, the virus rears its head again, and the UK government sends a team led by Major Eden Sinclair into the quarantine zone to see if maybe, just maybe, the few ragtag survivors there were have a cure of some kind. You can guess how well that goes: it is the kind of movie where you meet a lot of characters at once and in a group and so have no freaking clue what all their names are, but it doesn't really matter since most of them will be brutally dead in a few minutes. The movie changes tone radically a number of times, from gory horror to action thriller to gonzo comedy; it's the kind of thing Troma would make if they had a huge budget, and it's a testament to the skill of everyone involved that you actually make the transitions with 'em. "Yeah! Mobs of punk rock cannibals! Okay! Oh, look, a dance number! A locomotive chase! BRING IT ON, MOVIE!"
DOOMSDAY isn't bad -- it's a pretty decent B-picture -- but odds are you'll stumble out of it wondering what the *hell* you just watched. Don't ask me. I'm still dizzy.